How much vacuum are you pulling? It may be air in solution in the fluid bubbling out. If it's air getting in, the most probable place is the bleeder screw threads. I loosen mine about a third of the way out and rub a very small amount of Teflon paste around the threads next to the caliper/wheel cylinder, and then run it in and leave it "cracked" just enough to bleed. I have used vacuum bleeding before. Now, I almost always push fill brakes from the bleeders back to the master cylinder with a thumb pump type oil can with a piece of tubing attached to the bleeder. This eliminates need to bench bleed the master cylinder and pushes the air from the lowest points in the system to the highest. You do have to Teflon paste the bleeders to keep the system sealed while pumping it in, though.
