When you are "On boost" you aren't goping to be braking, when do do want to break you take your foot off the accelerator which will cause your biult up incoming air pressure (boost) to drop rapidly (within a second) and it will then be in the vacumm stage to help assist you brake.

Timing should be retarding when you increase your intake pressure otherwise detonation sets in so most turbo'd cars have thier dizzy locked at a certain timing, otherwise it's computer controlled.

Anything else that runs on vacumm in a turbo charged car is designed to only serve a purpose when your foot isn't on the accelerator like brake booster.... Etc

I hope I've answered your questions

Cheers
Damo