With a vacuum you are limited to the pressure of one atmosphere = 14.7psi. With a compressor supplying pressure you are only limited by the strength of the mould and the amount of pressure your compressor can supply.
When you blow up a bag with a compressor the excess resin is squeezed out the join in the two halves of the tube mould. You can vary the resin fibre ratio to some extent with the amount of pressure that you choose to use. I have made tubes with a pressure of 50 psi but they tend to be a little resin starved. In the past I have found 35psi to be about right for my purposes.
True and I agree 100%. Except, the vacuum has advantages with respect to removal of water vapour etc. The ideal is actually to do both (like the aerospace autoclave operators do). Create a vacuum and then apply additional pressure. This can be achieved in this case by putting your whole setup in a sausage back as well. The once everything is set up you "suck and blow".
