Only ones I can think of would be Battlefield: Vietnam, which is an early 2000's game, so probably doesn't have many servers available for it anymore or if you can even buy it still. Battlefield: Bad Company 2 had a full Vietnam expansion pack and since Bad Company 2 is still actively played, that might be an option. Probably the best overall would be Rising Storm 2: Vietnam, it's a more hardcore/realistic shooter compared to Battlefield, but it's similar in scale, having some vehicle action, but servers have like 40+ players on them and is actively played. I put a bit of time into it. Bad Company 2 is the only one on console, but I can't imagine there are a lot of folks playing it on there still compared to PC.