When? The precursors of the Viet Cong---then called the Viet Minh, if I recall correctly---did cooperate with the OSS against Japanese forces in Indochina. But then again, we collaborated with Stalin as well back then. Now, I'm not saying Ho Chi Minh was as bad as Stalin; I'm merely pointing out that during the Second World War necessity had us making deals with some shifty characters.
The argument that the NVA and VC were some kind of freedom fighters is shifty at best considering how they were fighting for a regime which was at least as bad---at the time---as the South Vietnamese one. But then again, Ho was a master at using nationalism to support his cause, which technically he wasn't supposed to do if you were following strictly the communist version of dogma.
This was a fascinating article, for all who are interested:
8 Things Vietnam War Movies Leave Out (By an Enemy Soldier)
But all things considered the Vietnamese Communists are nowhere near as nasty as say, China had been in terms of cracking down on personal freedoms.