Well...
A vacuum bleeder is just the opposite of a pressure bleeder, and you use it on the opposite end that you would a pressure bleeder. It creates a vacuum at the bleeder valve of the wheel cylinder or caliper and sucks (OK my physics teacher would be mad - allows atmospheric pressure to push the fluid from the reservoir) the old fluid out of the system. I've always preferred this to pressure bleeders since brake fluid is hygroscopic and my "new" fluid could end up sitting in the pressure container for some time before I actually end up putting it into a car. The vacuum bleeder always allows me to pop open a new bottle of brake fluid when changing the fluid. Hope that helps.
Bob