Frequently, the mould is on a plate which is moved up to push the mould
through the plastic on small vacuum formers. The mould plate is mounted
above a tank of the same area, but about 3x the depth of the plate
movement, which has previously been evacuated to low pressure, .05bar.
The action of moving up the plate, allows the evacuated tank to equalise
with the base of the mould which has vent holes inserted to aid airflow
into the vacuum. This action provides about a +0.25 bar vacuum. I think
this is rather more than the average shop vac. Many shop vacs use two
bypass motors(no cooling problems) to give better "suck", but as these
are a fan action, the suck will disappear if the airflow is blocked.
(AIUI All fans only operate by sucking, so if you block the airflow
you've had it!) If you wish to find out how well units suck, you need to
do a search using " wet & dry vacuum water lift" The norm is in the
region AFAICS of 7' which is roughly +0.8bar. If you are simply heating
the plastic and then sucking down at the same time, the temperature
control is very, very critical, too low and the material splits/doesn't
form properly, too high and you get holes everywhere! The process is
also dependent on the prior history of the plastic, who made it etc and
a lot of suck it and see is required.

Good luck

Capitol