Replastering is tricky work, so I cut the hole in the plastr to a square using a utility knife, with the edges of the square halfway across the stud, then I cut a piece of drywall to fit an then I tape and mud it in the same way I would a patch in drywall.
As for a vapor barrier, I can only suggest BIN Primer. Fair warning, BIN sets up fast and has a strong odor. You'll need to ventilate while you use the stuff or use a carbon respirator. I have had some problems with BIN releasing from the surface because of moisture built up under it, as the stuff doesn't "breathe", wish is a material's ability to pass moisture through it so it doesn't collect under it.