Plastering over wallpaper could work if it involves wire mesh, but this sounds like a skim coat and crossed fingers.

You can always go over existing surfaces by securely fastening the substrate to the framing or masonry. Or just take all the PO's "improvements" off and start anew.

I'm sure it was not done "the right way" if there is such a thing... you can see the wall paper patter through the plaster in some cases and the wallpaper seams, and in one room, the wall paper is starting to come up on a corner.

Any advice on how to go about taking it down?

Lots of wetting, scoring and scraping. I've found dishwashing soap works as well as the chemicals. I've never tried steaming or some of the other methods mentioned in the link.

Here is a link that might be useful: Gardenweb: Removing wallpaper

Steaming is still the fastest way to get wallpaper that is not strippable off the wall.

Even the strippable stuff sometimes leaves enough behind to make a quick steam pass worthwhile (usually idicates the walls were not painted or sized before papering).

The worst is when someone puts contact paper on walls.
A skim job (at least) is going to be required to repair the face of the drywall when it peels off with the contact paper.

Using white glue (like Elmer's) to fasten down edges also damages the drywall face.

But how do I remove the plaster that's on top of the wallpaper?

I had some patching done over wallpaper and I chipped it off until I could get underneath it and then got a putty knife under it and pried. It wasn't adhered well to the paper so it popped off in bigger pieces but it was still a bit of work.

"But how do I remove the plaster that's on top of the wallpaper?"

Take a wet spoge and see if the material softens and comes off.

Pre-mixed drywall mud sets by drying out.
If you wet it again it will soften up and come off (AKA 'wet sanding').

Basically you just wash it off.

Paint will slow things down.

If the surfaces are drywall it is likely to be just as fast to remove the wall and replace it with new.