The Vietnamese are essentially a team civilization with many bonuses that benefit their allies. They also feature strong anti-archer capability with the Imperial Skirmisher and the Rattan Archer, making them a fearsome opponent to other archer and cavalry archer civilizations.
The Vietnamese is a strong archer civilization, having access to every unit and important unit upgrade at the Archery Range, except for the Hand Cannoneer and Parthian Tactics. The most noticeable quirk of the Vietnamese is that they are an archer civilization with strong anti-archer capacity. The Imperial Skirmisher upgrade is a unique unit upgrade for the Elite Skirmisher, and their unique unit, the Rattan Archer, is an archer with high pierce armor and reasonably high attack. This allows the Vietnamese to win out in many archer vs. archer fights, even against the Chinese Chu Ko Nu. Their Archery Range units get more HP, allowing Vietnamese archers to survive more shots from Onagers. The Paper Money tech makes the Vietnamese one of the most reliable teammates, since they grant each ally 500 gold, hence boosting the team's economy. One of their civilization bonuses reveals the location of the enemy Town Center, hence allowing more scouting near the Vietnamese's own base and adjusting their strategy depending who the flank and pocket position players are in team games. The Vietnamese also get free Conscription, meaning the Vietnamese save a significant amount of time researching the tech and allowing more time used to either create more Rattan Archers or upgrade them into Elite. They also have access to every important unit upgrade for their infantry and cavalry lines, only lacking Hussars and Paladins. Finally, their Battle Elephants are more durable thanks to the Chatras unique tech, combined with their access to Bloodlines.
While the Vietnamese have clear strengths as an archer civilization, they do have a few clear weaknesses. Their infantry and cavalry frontline will not be as strong in offensive power due to the lack of Blast Furnace. Their Battle Elephants and cavalry units will be less mobile since they do not get Husbandry. Their siege line is also one of the worst of the Southeast Asian civilizations, lacking the Siege Onager, Siege Ram, and Heavy Scorpion upgrades, meaning the Vietnamese need to rely on Trebuchets and Bombard Cannons with Siege Engineers for their siege capacity. Their defensive capacity is also incredibly weak, lacking Masonry and Architecture, meaning the Vietnamese have to rely on Castles (where they get Hoardings), Bombard Cannons, and their army composition to play defensively. Unlike the other Southeast Asian neighbors, they do get Arrowslits, meaning garrisoning Arbalests and Rattan Archers in Keeps is always an option for offensive and defensive play.
While the Vietnamese are a strong late-game Imperial Age civilization, they are a team dependent civilization, as most of their civilization strengths are more team-oriented. This means that the Vietnamese are weak in 1v1 situations since their Paper Money unique tech will not be as strong. Aside from that, the Vietnamese do not have any significant early-game economic bonuses, apart from the revelation of the enemy positions at the start of the game, and most of their civilization bonuses only come into play in later stages. This makes them incredibly vulnerable to Dark Age and Feudal Age rush strategies, especially from stronger early game civilizations such as the Vikings, Huns, Mongols, Aztecs, Mayans and the Malays.
At the start of the game, the first priority of the Vietnamese is to play defensively, since the Vietnamese are vulnerable to early game rushes. Since the enemy Town Center is revealed at the start of the game (in nomad-style maps each of the first enemy Town Centers is revealed after being built), this allows the Vietnamese to scout their own base more than scouting the enemy base. This bonus is also useful in adjusting to their own strategies based on the location of the enemy player in both solo and team games alike (for example, if the Vietnamese is in a flank position while the enemy flank is the Huns or the Aztecs or if the enemy is much closer than expected, the Vietnamese should prioritize gathering Stone and placing defensive structures).
When the Vietnamese reach the Feudal Age, the Vietnamese should play defensively as usual. Building Archers and Skirmishers are recommended to take advantage of the Archery Range's HP bonus. However, if the enemy tries to do a Scout rush, it doesn't hurt to throw in a few Spearmen to defend the Archers. The Vietnamese should also consider doing some raiding with their Archers and Skirmishers, thanks to extra durability of their Archery Range units, and their knowledge on the opponent initial location.
During the Castle Age, the Vietnamese bonus health for their Archery Range units will be much stronger, allowing their Elite Skirmishers and Crossbowmen to survive a Mangonel shot. The Vietnamese have another Archery Range unit option on the table: the Cavalry Archer. Even though the Vietnamese lack Parthian Tactics and Husbandry, a Vietnamese Cavalry Archer rush is viable, thanks to the additional HP bonus combined with their access to Bloodlines (as well as having access to the Heavy Cavalry Archer upgrade in the Imperial Age). That being said, the Vietnamese should continue to focus on building Crossbowmen and Elite Skirmishers and consider building a Castle so they can pump out their anti-archer archer, the Rattan Archer. Battle Elephants are another option in the table, thanks to the extra HP granted by Chatras, thus giving Vietnamese a viable meat shield to protect their archers.
The Vietnamese will shine greatly in the early-to-late stages of the Imperial Age. Archers will be the main backbone of the Vietnamese army, with Halberdiers and Battle Elephants as their main frontline to protect them. Since the Vietnamese lack Blast Furnace, their cavalry and infantry frontline should be backed with their durable Heavy Cavalry Archers, Arbalests, Imperial Skirmishers, and Elite Rattan Archers, with Trebuchets and Bombard Cannons as their main siege support. Paper Money should be considered in team games, since it will put a significant amount of economic value to the Vietnamese teammates. With that being said, they also truly shine in trash wars with the value of the Imperial Skirmisher upgrade, despite lacking Hussars and Blast Furnace. When gold starts to become scarce, the Imperial skirmisher gives to the Vietnamese an edge in the "Trash Wars".
Due to the revelation of enemy Town Centers bonus, the Paper Money unique technology and their team bonus ( Imperial Skirmisher available at archery range), the Vietnamese are perfectly suited for a "support" civilisation and can assist considerably many civilisations in team games. Indeed, they offer their allies a broad selection of supporting bonuses that can be utilized to maximum effect in various stages of the game.
For example, they can assist in trash wars tremendously by providing Imperial Skirmishers to themselves and their allies; civs with already powerful trash (such as the Berbers, the Magyars or the Malay) or those with archer/skirmisher specific bonuses (such as the Aztecs' Atlatl upgrade and Incas' Andean Sling) may abuse this advantage greatly and acquire an alarmingly good trash army. A Byzantine-Vietnamese alliance can, in particular, prove to be truly beneficial, with the Byzantines benefiting from cheaper Imperial Skirmishers and Paper Money, facilitating thus the acquisition of Cataphracts that in turn counter civs with infantry with high pierce armor such as the Goths. Cheaper spear units can also help with early cavalry rushes in the Feudal Age.
The Teutons can also benefit from superior Skirmishers to counter gunpowder and cavalry archer civs such as the Spanish and Mongols, respectively. The Vietnamese archer armies can help against factions with strong anti-infantry units (e.g., Japanese, Aztecs), and Paper Money can be used to maximum effect (probably to train Paladins, Monks or Teutonic Knights). They will in turn offer great siege and fortifications as well as an anti-conversion boost to shield, among others, the Vietnamese Battle Elephants from Monks.
The Britons' team bonus may be combined with the tougher Vietnamese missile units and the Imperial Skirmisher team bonus to spam missile units at all stages of the game. The Saracens' bonus damage against buildings will make the Rattan Archer far more threatening to enemy towers thanks to the unit's natural high pierce armor.
Revelation of enemy positions may not seem as the most powerful bonus but it can prove useful when an ally is intending to rush an opponent as the Vietnamese player can address the exact location of the opponents. In Feudal Age thanks to Cartography, the LOS around enemy Town Centers will be shared between allies. Factions with stronger early game in particular (especially cavalry civilizations, such as Mongols) can abuse this fact to accurately rush enemy positions. The Portuguese are also a valuable ally, as free Cartography at the start of game can reveal enemy Town Centers for all the Vietnamese allies.
It should be noted that Paper Money can benefit any alliance (particularly ones with more than two teammates), but gold-hungry factions, such as the Spanish can be really boosted. As much as the Turks may benefit from the Paper Money tech, the Turks have incredibly poor synergy with the Vietnamese since the Turks do not get the Imperial Skirmisher upgrade (since the Turks don't have access to Elite Skirmishers) and the Turks' team bonus of gunpowder units created faster only applies to the Vietnamese Bombard Cannons, and the Vietnamese themselves do not get Hand Cannoneers. Conversely, the Berbers is also a strong ally to the Vietnamese, as the Vietnamese will also get the Genitour unit at the Archery Range. The Vietnamese Genitours not only benefit from the Vietnamese bonus HP civilization bonus, but further boost the Vietnamese anti-archer capacity with their Imperial Skirmishers and faster produced Rattan Archers thanks to Kasbah. The Berbers stronger early game can also offset the Vietnamese weak early game as well, and the Berbers accessing the Imperial Skirmisher upgrade is a nice addition to the Berbers' already strong anti-cavalry archer capacity with Camel Archers and Genitours. The Italians are also a nice ally for the Vietnamese, since both the Vietnamese and Italians can get each other's team-bonus-based unique units.
To conclude, Vietnamese may struggle in 1v1 situations, especially in the early game and particularly against rushers, but their excellent direct and indirect support should never be underestimated and indeed, they might be among the best civilizations, as far as team games are concerned.