Common Sense is one sourse. Since the fall of the Soviet Union, Vietnam has lost its security blanket...its protection from any possible invasion from the North (China). Now, without the USSR, there is no balance of power in Asia. China is IT! So, where else would Vietnam go?
There are several sources out there. I've also read in a few places that the US has been allowed to reopen an old base in Vietnam but mostly to store equipment for any possible war with China (if they were to try to invade Vietnam). This base is used more for storage than it is for actual US military personel.
Another concern of Vietnam is Al Qaeda and the Islamic Radicals in Indochina. US and Vietnam are also training together to fight the WOT in the region.