The Vietnamese would need lots of fighters just to match the Americans. The Tomcats would be a real problem, too, because those can launch from carriers.
If I'm the Vietnamese, I'm keeping the Su-15s and Yak-28s up as much as possible, and getting AWACS support. The Russians had figured that out by 1985, and the Vietnamese would need the help. They would also need the MiG-29 or Su-27 if they didn't want to be way behind on the fighter problem. The Foxbats are good, but not that good.
The B-52 has a big problem in that it absolutely requires air superiority, because its huge and slow. A great plane if you wanna blow up whole city blocks, but not good otherwise. That said, on airpower the US has a big advantage. If they wanted to press that advantage they'd bring out F-16s to smoke the radar/SAM sites.
If the US was serious, they would also try to get the B-1s into the action. That would be tough to do because they weren't ready yet in 1985, but they were close, and I imagine if this war a real fight they'd want the Lancers to take on bombing runs where the bombtrucks were just too fat to pull them off.
Another big problem for the Vietnamese - actually, probably the biggest in an air war - is the AWACS and the range of American missiles. The AIM-54 missiles have a range of better than a hundred miles, which combined with AWACS means the American fighters can shoot from long ranges. If the AWACS can be taken out, the Foxbats' powerful radars can make it a pretty event fight, because the American fighters would have to get a lot closer, to the point where the AA-11s can start making problems, particularly for the F-4s.