Do I need to add plywood underlayment over plywood subfloor?
I prefer underlayment On top of sub flooring if there are a lot of bulges from the screws I would put a thin layer of spackel or self leveling cement down then the sub flooring. 
If you use a pad the vinyl plank may be fine on the existing Sub floor but when redoing my houses I want it nice so I would use an underlayment. 

The problem with plywood is knot holes and blows will end up showing through over time with some floors underlayment is cheaper and better for this case. Always stagger the seams don’t put any new seams over the old ones.