I had noticed the poor performance of Vietnamese. These changes could fix it. But it would be a 50% re-work of the civ.
Vietnamese:
* [RETAIN] Reveal enemy initial Town Centers location at the start of the game.
* [REMOVE] Economic upgrades cost no wood.
[REPLACE] Receive 100 Wood and 100 stone arriving at the next age.
* [CHANGE] : Archery range units except skirms 20% more HP.
* [NEW] Battle Elephants, Rams and Siege Towers +50 HP.
* [RETAIN] Conscription is free.
* [REMOVE "Chatras" 100+ HP] Replacement: Castle Units and Galley line 25% cheaper.
[REMOVE "Team Bonus" ] Replacement team bonus: Fishermen and Fishing ship carry +10.
[BUFF] Imperial Skirmisher is made a regional unit of 'Rise of the Rajas' civs
I believe the problem with the Vietnamese is too few resources at early stage and no recurring bonuses as the game progresses. They don’t need imperial skirmishers since their normal foot archers are just as good as equivalent skirms in anti-archer role. These changes along with free conscription should help to push this civ up a notch.
Here is a link to that thread.