The wallpaper may be keeping the plaster sound. A previous owner may have put it up due to cracking and flaking and if I were you, I'd leave it alone. You'll, no doubt, end up gouging the top coat of plaster and requiring a new skim coat of plaster, which is not the same as finishing drywall. Henry's recommendation to install 1/4" or 3/8" drywall is by far, the easiest, cleanest, and less time consuming and will result in a beautiful new finish, instead of exposing a surface you'll learn to hate very quickly. Good Luck.