Vacuum forming is a process of changing plastics from one shape into a different shape. It involves heating a piece of plastic until it becomes soft and pliable. Then the pliable plastic is pulled over a mold. Vacuum remove the air between the plastic and the mold allowing the plastic to conform to the mold.
You have to us a thermo plastic such as ABS, acrylic, polycarbonate or styrene.
Thermo sets are a group of plastics that are not vacuum form-able. G10, phenolic and FR4 are thermo sets.
Some plastics like Delrin and Teflon out gas when heated and are very dangerous to vacuum form or even heat.