Although I've not seen it done myself I have spoken to a plasterer who recommended plastering over wall paper in this situation. Although it would obviously depend on the wallpaper and it would need to be prepared to ensure the plaster won't fall off.
Hi, I would say as you want to emulsion the walls to go along with acsimpson on this one and get the walls skimmed first then you will have good walls to paint and will not get this problem in the future. If you are going to do it yourself, remove any loose paper then give the wall a coat of PVA, allow to dry and then skim. I have done it on a number of occasions and it is worth it for the finish you get and the time it will save. Just don't know why they do this as it causes so many problems when you come to redecorate. Cost I suppose. Good luck with it. Shed.