I was thinking about this today and, forgetting differences in equipment, one thing that really says "Vietnam" to me is helicopter insertion into an LZ where you don't know if there are hostiles around or not. To simulate that, you could have a truck (or two or three) drop one team off in a clearing. Their mission is to patrol to a village and do something (kill a tax collector and take his bag of 'money' or something).
They jump out and lay low while the truck(s) leave, then have to make their way to the village. The enemy may be waiting in the treeline at the LZ, may ambush them between the LZ and the village or may all be waiting in the village. If the people in the trucks didn't want their vehicle to get shot, just make a rule that says they have to be off the field before shots can be fired. That's accurate enough, the NVA would sometimes wait for the helicopters to leave before opening up on the Americans.
One problem would be that the Americans may be slaughtered out in the open LZ, but you could put rules in place to give them a chance, like saying that 50% of the enemy force has to stay in the village. Or you could say that after the LZ battle is over, everyone gets to respawn. That's not very realistic, but otherwise, the NVA team will all concentrate on the LZ and wipe out the other team and the scenario goes nowhere.
There are various other ways around it that I thought of too, and you can get into some long and fun scenarios if you make the Americans an outnumbered Special Forces team.