I have a 2000 Ford E450 Aerotech bus but mine doesn't have a "vac" light.
The 7.3L diesel engine has a vacuum pump which feeds the power brake booster and those pumps have a reputation for failing. If your dash brake warning light is coming on and/or you have to stand on the brake pedal to stop, that pump is likely your problem.
Probably best to just call Goshen and ask what that light is. http://www.goshencoach.com/
Grammar: The difference between helping your Uncle Jack off a horse and helping your uncle jack off a horse.