you have to consider what the vacuum is doing.
If you are vacuuming material before pouring it into the mold, then you are degassing the material.
If you are subjecting the mold to vacuum after filling it, then you are lowering the pressure on your casting material inside the mold- this will cause all the air bubble in the material, and trapped inside the mold to expand dramatically, increasing their buoyancy so they rise to the surface, burst and are expelled by the pump.
At that point, if you leave the mold under vacuum, you still have bubbles trapped inside the mold- they are just at very low pressure.
What you want to do is vacuum the mold, and then, while the material is still liquid, release the vacuum… this will slam 14 lbs of pressure back on the material and cause any remaining bubbles to be squeezed either microscopically small, or into solution.
It is essentially like casting under pressure- you are just degassing the material and reducing the pressure of the remaining bubbles before re-applying a full atmosphere.
To that end- a couple of cautions…
Number one- it takes much longer to evacuate a chamber than to simply slam 80 psi into one.
As a result, you must make sure you are casting materials that stay liquid Long enough to allow you to attain vacuum, hold it long enough for the air to boil out, and then release the vacuum.
For that reason, Most urethanes will not be suitable- they set too fast.
Most vacuum casting is done with epoxies or polyester resins that give you at least 15 minutes of working time.
The second caution is that materials place under vacuum will triple or quadruple in volume as the air in them expands. this will cause resin to boil OUT of the top of the mold… and bubble for a while… so if vacuum casting, you need to make sure your molds are designed to accommodate this overflow and hold the resin over the gate so that as the air bubbles out, it can flow back down into the mold.