Ive grown up around Volkswagens, Ive owned numerous, and know the history of them pretty well. I looked at the set of keys posted in the link and judging by what i saw there was only one key i noticed that resembled something that could be used as a volkswagen key. For most Volkswagen replacement keys, the Ilco Key company usually has a blank for most VWs prior to 1999. In 1999 VW went the "switch blade" key fob which cannot be reproduced by Ilco. The basic cars that stand out are the VW Jetta, VW Golf, VW Passat..those three were the Big 3 in terms of sales in the US between the late 80's and 1999. from the late 80s to early 80's there were a number of vehicles, some more successful than others, again that included the VW jetta, and VW golf, with a few other vehicles thrown in such as the Rabbit and Cabriolet (which was successful) less successful models (which would be harder to come by in later years especially in the late 90s would be the Scirrocco, Rabbit pickup truck, Fox, Dasher, Corrado. I would highly rule out the VW Thing, Beatle, or Vanagon, or even the Kharman Ghia.
In my opinion, given the reproduction key, it seems likely it was from a VW Jetta (years 1986-1998) Golf (years 1988-1998) Passat (1990-1998) Rabbit (years 1983 to 1988) Thats not saying it couldnt be any of the others, just those were more common of a vehicle to come across, especially with Ilco reproduction keys.
I wonder if Law Enforcement looked into Missing/Stolen/Impounded/Abandoned vehicles in the area or surrounding areas.
Anyway, that's just a little bit of info