Vacuform kits are the simplest form of making kits. It's no different than the blister packs you find around toys, small electronics, headphones, tools and the like. A vacuum table and a heat source and a form, which can be another kit or a block of wood, is all you need to make a vacuform kit. Heat the styrene, turn on the vacuum and suck the air out which draws the heated stryene over the form. Trim out, sand to fit and you have a kit. The vacuum table can be made at home with a simple box, with the top drilled out and hooked up to a vacuum. Here's an article on it showing how simple it really is.
Vacuform kits are sometimes the only way to build a rare aircraft. Manufacturers aren't interested in investing in an extrusion mold (that can cost upwards of $100,000!) for a kit only a handful of people are interested in.
Vacuform canopies are much thinner than extruded styrene and are much truer to scale thickness. They are also an inexpensive way of updating one version to the newer one. Often times that is all that is the difference in model variations.
They also come in handy when you goof up painting or accidently cut or break canopies. ![Banged Head [banghead]](/emoticons/icon_smile_banghead.gif)