I don't think epoxy putty would stick very well to the vinyl and most epoxies are very rigid which could cause problems with thermal expansion during the summer. I have had the most success using the insulation foam sold at Home Despot. I think it is called Great Stuff. I open up the cracks like you describe and inject the foam, then quickly cover the cracks with masking tape to keep the foam in place. A word of caution this stuff is very sticky and it keeps expanding and can get all over the place if you are not careful. I would start with small area and see how it comes out before going wild. Latex gloves are absolutely necessary. Of course the other way of tackling this is with one of those plastic covers you can find on evilBay.

__________________
Bill Reimels
Now down to one:
1972 300SE 3.5 W109 (Euro delivery)