That is a "hornets' nest" of a question!
One thing is for sure, if you are using alumilite, you don't have time for both so just use pressure. For PR it is a tougher question. I use both every time I cast PR but others only use pressure and their blanks turn out fine. My opinion is the only time you would NEED vacuum is when you are trying to fill nooks and crannies. you want to suck all the air out so when you release the pressure the crannies suck the PR deep into them. Then the pressure just has to compress little tiny bubbles rather than a "bubble" the size of your cranny.
GK