The Vietnamese are a Southeast Asian civilization based on various dynasties in Vietnam before the European colonization. They are featured in Age of Empires II HD: Rise of the Rajas as an archer civilization with decent teamwork and team bonuses (a trash unit upgrade, a 500 gold tribute to the entire team and the revelation of enemy positions).
Historically, the Vietnamese were known as one of the few nations who successfully repelled the Mongol invasion in the 13th century. This is mostly attributed to the resistance of the Vietnamese army that mostly consist of peasants and volunteers that were quickly deployed in the battlefield to resist against enemy invasions. To reflect on the Vietnamese peasant army, they get free Conscription once they reach Imperial Age. The Vietnamese were also masters of guerilla warfare, using the Vietnamese jungle and cliffs as their advantage for traps and striking their enemies through the shadows of the Vietnamese jungle. Such guerilla tactics were used against enemy invaders and occupiers that withered down the enemy forces and the guerilla tactics were even used against the American troops in the modern times during the Vietnam War. To reflect on the Vietnamese mastery of guerilla warfare, they have a unique upgrade for the Elite Skirmisher, the Imperial Skirmisher, which is available as a team bonus for Vietnamese allies.
The Vietnamese were known to be skilled in archery, and one of their known bows were made of rattan, a light-weight wood which was historically used to make armours and shields, to boot. To reflect this, the Vietnamese gain extra HP for their archery range units and their unique unit, the Rattan Archer is an archer with incredibly high pierce armor. The Vietnamese were also known to breed elephants that were larger than most other elephants in the region, and deploy them in battle, which is reflected with their unique technology, Chatras, which gives extra HP for their Battle Elephants.
Even though the Vietnamese have resisted the Ming Dynasty's occupation and overthrew the Ming rulers in Vietnam, China has a strong influence in the region that shaped the Vietnamese culture. To reflect on China's influence, their unique technology, Paper Money, is a team-based unique tech that grants each Vietnamese ally gold. Finally, the Vietnamese relied on peasant spies and infiltrators to locate enemy camps in order to setup surprise ambushes in the enemy camps. This is reflected with their civilization bonus where enemy Town Centers are revealed in the start of the game.
The Vietnamese are an Archer civilisation with 3 team bonuses that can help them shine in situations with multiple players. They are the closest to a support civilisation in Age of Empires II, as their advantages are evident when they play in a team. To begin with, they can reveal enemy positions at game start, which combined with Cartography reveals enemy Town Centers to the entire alliance, especially beneficial to rushing tactics. In the Imperial Age, their status as a team civilisation is further cemented, as they grant Imperial Skirmishers to the entire team, and they can also research Paper Money in order to provide a gold tribute to all allied players.
As far their military is concerned, they draw their main strength from the Archery Range; they have access to all archer units except Hand Cannoneers, and only lack Parthian Tactics among archer-related upgrades. However, this is not too much of a loss since Parthian Tactics is made for Cavalry Archer-based civilizations. Their Archery Range units receive extra HP per Age, allowing them to withstand more attacks and fight back longer, at all stages of the game. Cavalry Archers are especially boosted, as they boast HP second only to Sipahi-boosted Cavalry Archers. Their first unique unit is the Rattan Archer, an archer with high pierce armor, decent speed and a good attack that allow him to withstand units with pierce damage, such as Scorpions and Arbalests, and fight effectively other archer units. The second is the Imperial Skirmisher, an upgrade to the Skirmisher line, with slightly improved stats, that can provide an advantage in trash wars to the entire team, being their main Team Bonus. They also have complete infantry lines, but they lack Blast Furnace, and their cavalry is unremarkable (lacking among others Husbandry), relegating their melee units into support role, mainly for guarding missile units. Conscription is free in the Imperial Age, saving some time and resources, while also giving a Vietnamese player a speed advantage to quickly train some extra units. Their Battle Elephants are used as meatshields, rather than offensive units, with the Chatras technology granting them a +50 HP bonus, in order to better shield missile units against units that can harass them. They have a below-average siege, lacking all final upgrades to their siege units (but receive Siege Engineers and Bombard Cannons), and their structures are tied with the Aztecs' for the frailest in the game (lacking Masonry and Architecture), necessitating use of archers in defence, though at least, they receive Arrowslits, Bombard Towers, Keeps and Hoardings, meaning they will still boast powerful if fragile base defences.
Because of the lack of good early game economic and their civilization bonuses as well as Paper Money is really team dependent, the Vietnamese is not an ideal civilization in 1v1 matches, especially against civilizations with strong early game rushing potential such as the Aztecs, Huns, Malay, and Vikings. Conversely, the excel in teamgames and are a formidable lategame foe to archer civilizations such the Chinese and the Mayans, due to their strong anti-archer capacity with Rattan Archers and Imperial Skirmishers.
Their in-game language is anachronistically modern Vietnamese, not Middle Vietnamese spoken during the feudal era (e.g. "heaven" is pronounced "trời" instead of "blời" as attested in the 15th-century's Quốc Âm Thi Tập-Anthology of Poems in [Our] Nation's Sounds by Nguyễn Trãi, Lê Lợi's retainer).
Dinh Bo Linh (Đinh Bộ Lĩnh) (924–979): Vietnamese emperor from 968–979, first independent ruler of a unified Vietnam since its liberation from Southern Han Chinese control by Ngo Quyen. Emerged victorious after the chaotic "Anarchy of the 12 Warlords" period.
Le Dai Hanh (Lê Đại Hành) (941-1005): Became Emperor of Vietnam after deposing the six-year-old heir to Dinh Bo Ling, ruled from c. 980–1005. Successfully defended his empire against invasions by the Song dynasty.
Le Thai To (Lê Thái Tổ) (1384-1433): Emperor of Vietnam from 1428-1433, first emperor of the Later Le dynasty which would remain in power for over 300 years. Came to power via the Lam Son uprising against Ming Chinese rule.
Le Thanh Tong (Lê Thánh Tông) (1442-1497): Later Le Emperor of Vietnam from 1460-1497; Lead Vietnam through prosperous times and military successes against foreign threats. Also reformed the empire's legal system.
Ly Nam De (Lý Nam Đế) (503-548): Vietnamese king from 544-548; first ruler of the Early Ly dynasty. Originally a regional leader within the Chinese Liang Dynasty's administration of northern Vietnam, he resigned, and with local forces, rebelled to establish his own kingdom.
Ly Nhan Tong (Lý Nhân Tông) (1066-1128): Vietnamese Emperor from 1072-1128; a ruler of the Later Ly dynasty. Established Confucianism as the state philosophy; created schools of Confucian learning.
Ly Thai To (Lý Thái Tổ) (974-1028): Vietnamese Emperor from 1009-1028; the first of the Later Ly dynasty. Worked his way to a prominent post within the imperial guards, enthroned by imperial guards after the fall of the previous emperor.
Ngo Quyen (Ngô Quyền) (897-944): Vietnamese King from 938-944 who rose to power after defeating Southern Han Chinese forces at the Battle of Bach Dang River in 938. His reign was marked by chaos and unrest, however, and his death was followed by the "Anarchy of the 12 Warlords" period.
Phung Hung (Phùng Hưng) (761-802): Mounted a rebellion against the Chinese Tang Dynasty in 791, becoming the de facto ruler over the region from 791-799. The Tang Dynasty still officially laid claim to the region.
Tran Anh Tong (Trần Anh Tông) (1276-1320): Vietnamese emperor of the Tran dynasty; ruled from 1293-1314. Reign notable for relative peace and prosperity, upholding a détente with the Mongol Yuan Dynasty. Son of Tran Nhan Tong.
Tran Nhan Tong (Trần Nhân Tông) (1258-1308): Vietnamese emperor of the Tran dynasty; ruled from 1278-1293. Presided over the repulsion of invading Yuan Dynasty forces on land and at sea. Father of Tran Anh Tong.
Tran Thai Tong (Trần Thái Tông) (1218-1277): Vietnamese Emperor from 1226-1258; the first of the Tran dynasty. Used guerrilla warfare tactics against the first invasions of Vietnam by Yuan Dynasty forces. Grandfather of Tran Nhan Tong.
Alongside the Berbers, the Vietnamese are the only civilisation that technically offers more than one team bonus (the Imperial Skirmisher, revelation of enemy positions and a 500 gold tribute unique technology), and the one with the most team bonuses (three, whereas Berbers have two: the Kasbah unique technology and the Genitour).
They are also the third civilisation to provide a unique unit to their allies, after the Italians and Berbers (though technically, the Imperial Skirimisher counts as an upgrade to a generic line and thus does not suffer bonus damage from Samurai, almost like the Indian Imperial Camel).
One of their unique technologies, Paper Money, is likely based on Ho Quy Ly's monetary reform in the 1400s, where he replaced metal coins with paper money as the mean of transaction.
Despite the Vietnamese using the Southeast Asian architecture, Vietnamese architecture in real life strongly resembles to Chinese architecture (or East Asian in AoE2's parlance) due to China's strong cultural influence in Vietnam throughout medieval Vietnam's history.
The Vietnamese wonder, But Thap Temple, is such a Vietnamese architecture's sample.
According to the developers, the use of Southeast Asian architecture for the Vietnamese is supposed to reflect on the Dai Viet and Champa kingdoms in South Vietnam.
However, developers' explanation does not satisfactorily address the issue of historical anachronism. The Vietnamese campaign took place during the Fourth Chinese Domination (1407-1427), & the post-Trần-Hồ's Dai Viet then did not include the majority of Champa territories; not until Lê Lợi's grandson Tư Thành would conquer two Champa principalities & reduce the third one to a vassal tributary. Even after the conquest, whenever Vietnamese settlers moved into southern territories they always built Sinicized towns (e.g. Hội An in former Champa lands).