Well, on the surface, absolute vacuum will only generate atmospheric pressure on the item in the bag...which equates to about 14.5 psi. You can try to pull "more" vacuum on it but once you have removed all of the air, there is nothing else to pull. So, the vacuum bag can only match the differential pressure between the outside and the inside of the bag...i.e. 14.5psi atmospheric and 0psi inside the bag = 14.5psi total...and you can't generate less than 0 inside the bag because there's nothing left to draw out.

That said, you CAN increase the external pressure on the bag to increase the differential and the pressure on the item inside the bag. This is the principle of an autoclave. You first draw a vacuum on the bag, insert the bag into a pressure vessel, pump up the pressure inside the vessel and put a lot more pressure on the part. Hall Spars builds masts this way (and the Autoclaves normally employ heat as well).

You ~could~ also take your vacuum bag and lower it into water to create more differential in the pressure in the same way the autoclave works.