Fuel tanks (including those on the Crown Vic) are designed to take some abuse. I would take it on faith that something out of the ordinary did puncture the tank or possible broke a fuel line etc. As we have no pictures it very hard to say what went wrong.

I would also suggest that the price the dealer is charging is extravagant. I'm not familiar with the VW's tank but most tanks can be dropped from the car rather easily. A used tank shouldn't be too hard to find and the installation can probably be done for a few hundred dollars. I would call around and ask shops about used vs new parts.