Thailand has a close relationship with Vietnam?
Sure, I guess you know your worst enemy well.
While things might have been warm immediately following the Communist coup in Thailand, relations quickly soured after the Thais made it clear that they weren't going to be a satellite in the vein of Laos, and that they were too powerful to simply invade like Cambodia.
The Thai-Chinese friendship is one of security for Thailand, a warning to Vietnam to stay away.
OOC: Despite the fact that they're both Communist! Vietnam and the PRC have never been friends.
The PRC even invaded Vietnam once.