The higher the vaccum the better off you are. This affects cooling time, and overall detail. Most hobbiest form by drape method. They push a male or female mold into a soft sheet and turn on the vaccum. Air pressure does a pretty good job of pushing the material down into or over the mold and seals until the plastic material cools enough to leak and then you loose vaccum. When you loose vaccum you loose detail, and when you go to strip the part off the part typically locks on the mold. Commercial formers use some type of perimeter seal on their mold to eliminate loss of vaccum and to allow atmospheric pressure to push down on the "plastic" sheet. If you are running 29 inches of vaccum or what ever, you try to maintain this thru the cycle. This mean you try to only evacuate the initial mold area and maintain this until you are ready strip the part from the mold. The perimeter seal can be made by using a clamp box with a mechanical seal or some form of o-ring locking the material to the mold surface. Other materials such as the oilefin families usally combine a moat groove around the perimeter of the mold along with a clamp box sealing to the mold surface.
Been there and done it for a few years.