John K Jordan
The solvent/vapor problem might depend on the type of epoxy and the vacuum pressure.
Years ago I operated a small metallography lab and daily potted a small stack of nuclear reactor element samples in epoxy, poured into a plastic ring. I put these in a vacuum chamber and the vacuum pulled the air out from between the stack of metals. The epoxy set up reliably. I don't remember the type of epoxy or the vacuum level since this was almost 50 years ago.
In Barry's situation I would start by contacting one of the major epoxy manufacturers and get the right epoxy. Vacuum should work fine if needed. The real problem would be effectively sealing the slab. If you can't seal it from the epoxy in the air it may be a challenge sealing it for the vacuum treatment.
The wood stabilization I've seen for turning blanks always submerged the entire piece of wood. That might be impractical for a large slab. Maybe ask Google for info on stabilizing wood. I saw lots of info including this: https://www.thebasicwoodworking.com/how-to-stabilize-wood/
My approach would be to start with better wood. Cut the other up if needed and use it for something else.