I find the manuals aren't too good at describung checks of the vaccum system, they just say check it then go on to electrical checks. Did you find that the brake (& clutch if applicable) pedal switch holds vacuum? You can pull the hose at the pump that runs to the pedals & pull vacuum with your mouth. Often the plastic hose under the dash will leak. Here's a link to a guy that has a different approach:
http://www.gti-vr6.net/library/elect...rol.html#mark0