I think Firaxis prefers Chinese culture to Indian culture, as well as Western Europe to Eastern Europe like someone has said in another topic. We have seen almost every western european Civs but Russia is the only eastern european civ included so far. The same goes for Asia, you have seen Mongolia, Japan, Korea, but not Pakistan, Bangladesh or Sri Lanka. So this time, the next asian civ will be Vietnam, who though shares a few things with China, but still represents the unique culture of the region.
Not like in China where there's no Mao in the game, no one in Vietnam will protest if Ho Chi Minh or whosoever picked as the leader doesnt look as good as they want. And you still can play in Vietnam games like Viet Cong, Battlefield: Vietnam and other games with Vietnam in the title (most of them are crap, but people still buy them because of Vietnam). Firaxis has added Korea for commercial reason and they will add Vietnam as well. After all, Thailand is too peaceful to be included in a war game