For me this could go under resin casting or modeling. Vacuforming I guess is more modeling so I guess I will put in that category. This is my first attempt and as with any first attempt you solve the problem with the tools you have in your toolbox. That said, my way is definitely not the only way or even the best way but it's a start using what I know. I will use two tools from my tool box and they are Milliput epoxy putty and resin casting to create the vacuform bucks for the vacuforming process. If this interests you then click the read more link.
For me the first thing were to create the masters for the mold. You can either replicate and existing glass or if you have modified the body as in the case of the VW/Squash Bug you will have to create the master. In the Squash Bug's case I had to create the master. To do this I used a product called Milliput. The glass for the Karmann Ghia is an existing molded glass from the Karmann Ghia. As a side note the nice thing about Milliput is I am able to file sand and polish to refine the fit to the body.
Once the masters have been created I then make a mold of them so I can resin cast and duplicate them to make the most efficient use of the vacuform plastic.
Once you replicate the desired amount which are now called the vacuform bucks you can lay them out on your vacuform machine and create the glass for your slot cars.