A Contract Emulsion is essential when painting new plaster, this is pure matt finish which allows any moisture to be released though the paint. A mist coat first (thinned paint by 25%) followed by two un-thinned coats. Most paint companies should only recommend vinyl matt or silk paints after 3 to 6 Months.
Check for any Knot, apply a Knotting Solution (help prevent resin oozing from the Knot), apply primer one undercoat and two top coats. This applies when using both oil or water-based systems.
Many use a steam stripper which does work but I avoid if possible. They are rather messy and can bring top surface of plaster off. We usually used hot water and washing up liquid, on the first wetting start soaking from the bottom of the wall working up and after that wet from the top (When the paper is wet it acts like a sponge and soaks up the water rather than running down the wall wetting the floor) Wet the walls twice leave 10 mins, wet again the wallpaper should be able to be removed easily if still difficult restart the process again it may take up to 6/8 soaks.