The Comp 292 cam you show isn't much different than the Mopar Hemi 292,510/512 cam used in the 4 speed cars.They usually carry 12 to 15 inches of vacume.I have used the Comp Cam in many street applications and have all carried good vacume to operate vacume assist brakes.
You have an issue that needs to found and corrected.Did you remove the pan from the bottom of the intake and check for leaks at the core plug or for any cracks?Did you check for timing fluctuation?