If wallpaper is one layer with primer...does the paper have to be removed or can plaster be applied over it?

I would remove it. Over time, wallpaper can loosen and peel (especially at the seams and along the ceiling line). Add the weight of plaster, and it will probably happen even quicker. I know that wallpaper is a hassle to remove, especially if it has not been sized. But your end result will be well worth the effort.

I agree with Amy, also I have seen people seal with primer
and it must be an oil primer not latex or it will bubble
even worse. I always remove wallpaper. Even primed could
be trouble, the amount of pressure from the trowel used
to apply the plasters along with the substrate absorbtion
will weaken the paper. Imagine the friction and pressure
from burnishing, especially Veneziano and your subsurface
starts to lift. No Goood.

Vallie

Artiste,
pls *always* remove wallpaper.
Safra products (both lime and syntehtic ones) grip very strongly to the wall, thus applying a certain amount of mechanical stress to the wall surface. If wallpaper has not been removed, the wall is going to create problems anytime between "very soon" to "quite soon".
Luca

If they can't afford to remove the wallpaper, they can't afford to have me plaster. I would never apply over wallpaper.
Doyle

I have done allot of wallpaper stripping in my growing up years working with my mom as well in this business.
The only time I would ever leave the paper on the walls is if it is over drywall, if the paper was not seal/sized by the hanger (you can tell) and if it will not come off whatsoever. This I run into every now and then. It is actually worse to scrape and wet over wet trying to remove the paper. It isnt good for the drywall. I would peal off the loose, seal with oil, sand, skim seams, seal with acrylic. This is last resort. If it's on plaster walls it will come off. If its a paper-backed vinyl it will also. There is that one type of paper that sometimes will not come off the walls if they werent prepped right.
Little secret: The Mur d' art pictures of the foyer I posted about a month ago, the one with the green louver cabinet doors,...that was over wallpaper.

Interesting Scott, because my brother seals up all
of his paper with oil then skims and seals. That is
the correct way if you are going to do that. If it is
a vinyl wall paper I can usually peel of the top layer
leaving only the brown backing, if sized I spritz with
my garden pump-up warm water and vinagar or diff then
it peels like very easily. All paper is different of course.
I took of 2 layers on a 60 year old house and it actually
came off good but had to let the rock dry out with fans.
The next day I still had to seal first 2 coats then float
out it was a major deal. Still have the pictures of that one. Vallie

Steve,
If the paper will absolutely not come off, your plaster is nice and sound and still in good shape, then I would refer to my previous comment on this thread. Needs to be sealed with oil though so you will not reactivate the old glue by any means. Once that is done, and the oil is thouroughly dry, what it will do is harden the paper and make it kind of "crispy". That is when you sand off all the liitle loose particles. Then whip up some mud and skim it out. most areas will take a couple coats. You can use quickset for the deep areas, and float out the last coat with EasySand.
Once that is dry, you may need to sand again (depending on how good you are with a mud knife) Then prime it with an acrylic primer thinned down with water to let it seep into the mud a little better.
Even after the acrylic is dry, I will sand again very lightly with a fine sanding block.
Your walls will be baby butt smooth.
This is what I do in these situations, someone else may have other ideas.
Luck