Not a vacuum FORM, a CHAMBER!
It would be a sealable Chamber. A tank to pull a vacuum in. After mixing the mold silicon you put it in the chamber, pull a vacuum on it to remove bubbles before making the mold and it becomes very smooth, no pits in the cast part.
What I have done is the easy way, but has to be repaired a bit because of the bubbles in the mold.
I'm filling and sanding. Had a big lesion on the tail pipe so I used Miliput to repair, that is out of reach and will be sanded down before priming and painting. It's going well!