Vacuum-forming has a number of purposes, such as making duplicates of small parts, replacing a broken part, creating an unavailable part, etc. The most common use is making a/c canopies. Why canopies? Well, most kit canopies would scale out to being 20 to 30 cm thick. A vacuum-formed duplicate is much closer to the real thickness. Being thin, they are usually clearer that the kit canopy, assuming good quality plastic. I recommend Vitax plastic. Check Micro-mark catalog. And yes, you do need the original part, or lacking that, you could carve a "master" out of balsa wood. Once you have a vacuum-former, you will wonder how you ever got along without one.
Tony Ryan