I have not done either but I have read tons of posts about both. My first thought when i read your question was that the pressure pot is used to reduce bubbles in castings. what I have ready in the past about vacuum (in regard to stabilizing) is that it creates bubbles, lots of them, as in foam.
from that my best informed guess is that vacuum is not a good idea for casting. but like I said I have never done either. and you woudl not be the first to set off down a road that seemed impossible only to prove otherwise.