I suspect they may have had at least a few pilots and aircraft rated for night attacks.
The North Koreans certainly did in the Korean War, so I imagine the VPAF, and/or their "allies" certainly did.
Most likely though, night operations by their fighter aircraft probably would have been fairly rare.
Flak, unless radar controlled, would certainly be limited by the lack of visibility vs. attackers.
There would not have been much, if any degradation vs. the SAMS though, since those were radar controlled, other than perhaps the inability to salvo missiles and use optical control/detonation of those, if their radar systems were jammed.
I hope that helps.
I can't recall reading much, if anything about attacks by Migs on US aircraft against night sorties, during the Vietnam War, but it's been a while, so there might have been some engagements.