Su-27s and MiG-29s would not have been in front line service until late 1984 at the earliest, and Soviet AAMs of the era (early-mid 1980s) were not in the same league as their western counterparts. And that's before we even get to the subject of radars and ECM.
If the Soviets had moved one of their Kiev-class carriers into the area, what would it have accomplished? As an ASW platform, it may have been of some use, but its air wing of Yak-38s would have provided target practice, at best.
Tu-22m bombers would have been cause for concern, with their ASMs...but moving them to them to front line airbases in Vietnam would have exposed them to attack from sub-launched TLAMs.
A more likely scenario was once envisioned by Tom Clancy (in his non-fiction book, "Fighter Wing"), and that is of a counter revolution taking place, where Ho-Chi Min city (Saigon) and the surrounding region secedes from Vietnam, and the US intervenes on behalf of the "new" Saigon government.