I was just re-reading this and made minor corrections--I mixed up pressure pot and vacuum chamber, which are two totally different beasts. Hopefully that didn't mess up anyone. My apologies.
Searching through the internet, I found this, which explains it in terms I can easily understand and retain.
Vacuum casting actually removes bubbles and air from your RTV, before you pour.. so you're bubble free in the mold. What pressure casting does it make the bubbles SO small they essentially disappear. some guys vacuum the rtv and pressurize the resin. there are many schools of thought on the topic, and most of them DO work. you have to pick what you can afford, and what works best for you.