I am currently going through a similar situation. Wallpaper on plaster walls, some painted with latex paint. Fortunately in our case the paint was not lead based so we didn't have to deal with that issue(check your local regs!).

Our process was to use a wall stripper (4" razor blade with long handle) to get as much of the paper off the wall as we could. In some areas/rooms we were able to get down to the bare plaster with no problem. Other areas/rooms were much more difficult with most of the paper backer being left on the wall. In both situations we wet down the walls with DIF (concentrate) in a garden sprayer and the remaining paper backer came off no problem, leaving us with bare walls.

Now that we have bare walls we're going to wash them down with DIRTEX, repair cracks and holes and then prime/paint the walls.

Good luck with your project and please let us know what you do! Hopefully you can save the plaster walls.