Don't glass on top of the vinyl as it just wont stick properly. You could have a hard job getting it to stick the plastic depending what type it is. To give it as good a chance as you can make sure you give it a good sand with some rough paper.
You can actually buy vinyl repair kits that will sort the cracks nicely. They come with embossed paper, you put the goo on the cracks then use the embossed paper that matches the original pattern. If done properly its an invisible repair.