You stated that you are able to get a solid pedal with the vacuum hose disconnected. If this is the case then it's doubtful you have a seal problem with the calipers or air in the lines. Both of these issues will give you a spongy pedal regardless of wether the booster is connected or not.
That's what I used to think too, but the same thing just happened to me!
I noticed metal shavings inside my front wheels a few weeks ago, and when I examined the brakes, I noted that even though the tops of the pads looked almost new, the bottoms of the pads had actually worn down so much that the metal backing was digging into the rotors. When I pulled everything apart, I noticed that the pistons in the calipers were "cocked", which allowed the pads to wear unevenly. So I bought new rotors, bearing, pads, etc... and put it all back together again, making sure to gently push the pistons back into their bores before I reinstalled. after that I blead the brakes, and everything appeard to be fine. However, as soon as I started up the car (and the booster kicked in), the pedal went all the way to the floor and I couldn't get a good feel. I shut off the car and blead them once more and they felt fine again. Until I started the car up and they went to the floor again! I went through this routine several times and although I could get a nice hard pedal each time I blead them, I could never get the brakes to work when the booster kicked in.
So I pulled the calipers and sent them back to Vette Brakes for evaluation, and they found a few bad O-ring seals and "rubber residue" behind the pistons. They also found one sleeve that was scored (probably because of the "cocked" pistons). They said that the worn O-rings were probably causing the calipers to suck in air, but they weren't bad enough to leak fluid out. Now, I have no idea why the booster exagerated the problem so I'm not positive the calipers were the issue, but it seems to add up because the master cylinder and booster are only a few months old, and everything was working fine before I installed the new pads.
I haven't gotten the calipers back yet (they'll be here early next week), so I won't be able to let you know what happens until they arrive and I put everything back together again, so stay tuned...