Air Conflicts: Vietnam is disappointing to me seeing as I really liked the previous Air Conflicts games. The previous games were all about fast arcade-style air-to-air and air-to-ground combat in large open arenas, and Vietnam basically gets all of those points wrong. The maps are very small and often obscured by low fog, so you're constantly going past the map's walls. The controls are fiddly and strange, nothing at all like the fluidity of Air Conflicts: Pacific Carriers (which I think is the high point of the franchise).
By far the worst thing about the game is that it interrupts gameplay to deliver cutscenes after the completion of each goal, and these interruptions are frequent. Your goal might be to take out three AA emplacements — which you'll get done within 1-3 minutes since the maps are so small — and then you'll sit through a 20 or 30 second cutscene with hammy voice acting. This will happen about five times per mission.
Give this a miss and try out Pacific Carriers instead. Secret Wars is okay too. I regret buying this game.