http://www.r2000manitoba.com/above_why.shtml
You'll need plastic receptacle vapour barrier pans (really cheap, way less than $1 for standard sizes), and a roll of red sheathing tape (Tuck tape is the most popular brand in Canada, not sure if it's sold under that name in the US).
Cut your VP, install your recept with the poly pan behind it, seal the wire penetrations with acoustical sealant or spray foam out of a can (from the outside not inside!), repair your VP incision with the sheathing tape, then use the tape to seal the poly pan edges to the vapour barrier itself. Fast, easy, super air tight, and the best way to do it.
If you're using airtight plastic receptacle boxes, you don't need the vapour barrier pan, and could get away with cutting a piece of VP enought so it straddles the two contiguous studs, run a rectangle of acoustical sealant on the face of the VP, place the cut piece of VP over it, and staple the cut piece onto each stud bay a few times.
But, I hate acoustical sealant. It's an awful mess, gets everywhere, it never dries, and it stinks. I much prefer spending a bit more money on the Tuck Tape method.
