When cutting vacuum formed pieces, I have (after much trial and error) found it best not to try to cut right up to the final edge of the part. Instead, I use scissors, snips, a sharp knife, or whatever is convenient to rough out the shape, leaving a small amount of material which I sand or file away to the final edge.

For larger parts, I will use blue masking tape to mark off the final line, then use a rotary sanding drum in my Dremel to clean up to the line. For smaller parts, I use a bit of sandpaper stuck to a popsicle stick, dowel, or some other suitable "stick". For very tight corners, I use a set of needle files for the final clean-up. If you need a good square edge (e.g.; two halves of a set of wheel pants), lay down a bit of sandpaper on flat surface and carefully slide the part on it (rather than sliding the sandpaper) for 2 or 3 light passes to square up the edge.