Think of it this way, use vacuum when you want the liquid to penetrate the wood. The vacuum will draw the air out of the wood allowing the liquid to get into all the tiny little nooks and crannies of the wood. Pressure will not remove the tiny little air pockets from the wood but it should compress it a little. That will force the material deeper into the wood. Also think about the viscosity of the liquid you're using. Stabilizing solutions tend to be very thin and less viscous. So if you remove the air pockets from the wood with a vacuum the liquid will naturally penetrate into the wood. If you have a thicker liquid and therefore more viscous, you can remove the air pockets from the wood but the liquid will still not penetrate into the wood on its own. You need to apply pressure to force the thick liquid into the wood.
I hope that helps a little.
