Go to a fabric store and get a vinyl repair kit.
I used a product called Glu&Fix from Bostik which is a clear adhesive for soft plastics, I think I bought it on e-bay, to stick back a welded seam on my plastic pacamac that had come apart. It worked well on that and I'm sure would on plastic pants.
clear plastic trash bags are great too.
You should use a soft rubber or plastic bonding glue (industrial strength). There are several of them available. I used to use Blue Magic waterbed glue, but it's hard to get now. I"m using a product called E6000. Any department store like Wal-mart, K-mart, etc has it.