There is no need to seal the walls with PVA, even if it was brand new plaster, people are under the conception that you must PVA new plaster all the time, you dont.
if you have taken the paper off and its down to bare plaster, the chances are the walls have already been sealed, or even sized prior to papering.
all you need to do is your prep etc, and use a thinish coat as your first coat, then paint your further coats as normal.
I'm sure Hammy will agree with that.
I only PVA big lumps of filler like I said before, I never PVA new plaster, I use a matt paint as a miscoat, and vinyl matt if its only a skim new plaster, but in your case old plaster, I would just do what I said above.