Unless this condition was present before the brake job I don't see how suddenly you would have a failed brake booster after. Anything is possible but that sounds unlikely.
What brand brake master cyl did you use? If the depth of the "cup" where the push rod from the brake pedal/booster rides is not identical to the master cyl you replaced this could cause this low pedal situation also. Scott mentioned something similar to this in an earlier post.
Also it's possible the rear brakes are incorrectly adjusted. Pull off the rear wheels and check the gap between the rotor and the pads. Should be a barely visible gap present. If a big gap is present here this will cause the pedal to travel some distance before engaging the rotor.
If it needs adjustment ratchet the caliper piston out using the parking brake cable arm at each caliper while rotating the disc until the pads are just barely touching the disc. If a shop did the brake job they should know this procedure and theoretically already have done it.
Best wishes,
Justin
2000 EVW 5-spd TDI
