A large cam will decrease vacuum noticably, with 220-230 degrees at .050" having around 12" vacuum at an idle. That amount of vacuum should be enough to run the power brakes.

I have used cams with 230/240 duration without loosing the power assist, even though Pontiacs with their 30 degree intake seats typically behave as though the cam were another 10 degrees "bigger" in other engines.

Most "streetable" cams will result in at least 8" of vacuum. Below that and you start loosing the signal needed to pull fuel through the carb accurately at an idle and at low speeds.

The intake has more to do with what range the engine likes rather than how much vacuum it makes.