It depends on what you want to do. Thailand for the sun and sea, perhaps some cheap stuff, fakes..branded stuff and fake women too. Korea, well, plastic beauty, cosmetics, expensive local tours, kimchi, barbecue meals. Vietnam, I can't say, haven't been there for such a long time.

Thailand and Vietnam are both in the midst of protests against either their local or the Chinese government.  I suggest the Philippines.  Good weather, lots of different places to visit, and overall outstanding English proficiency amongst the local population.  Or, these days, you could probably get a great deal for travel to Malaysia 

I would recommend Laos and Cambodia. It's cheap to travel there, it's quiet, it's clean, the locals are cool, English is reasonably spoken, lots to see and do, great food (and some Chinese and/or French). Vientiane is hypnotic, Siem Reap and the surrounding countryside is grat too, I heard great things of Sihanouk Ville

Vietnam is nice, but if you travel with your Chinese gf/wife, it might be dodgy, due to the strong anti-Chinese feeling (it's can be really ugly, same level with anti-Japanese feeling here). Thailand, pretty sure it's all fine outside of Bangkok. But there might be small chances of being stranded at the airport.

Vietnam does amazing bread and coffee, if you miss those, go there!! But be careful about rip offs, and stay away from Taxis, they are more expensive than China

Korea is really nice, but also kind of boring. 

Thailand, very touristy, you can find places that are more original, but generally there are tourists everywhere

Vietnam, of the 3 would be my pick.... but what about Malaysia. Kuala Lumpur is an awesome city and there is lots of places to go in Malaysia. Look into it.