Revving will probably reduce the vacuum. Just let off the gas as you come out of the burn out. That should give you plenty of vacuum. Don't bump the brake anymore than you have too as you stage and use the vacuum up as you are starting to build boost.
There is no reason not to be able to hold boost with vacuum brakes. Now, if your braking system is not in proper order that will not help. If your converter is not matched to the turbo, that will not help.
Both of my cars will hold as much, and actually, more with the vacuum brakes as they did with a PM. If you have 13" of vacuum or more, at idle. You should have plenty. If you think you are using up too much of the stored vacuum while staging, I guess you can use a reservoir or add a vacuum pump but it should not be required.
Now if you went overboard on a cam and don't have any vacuum at idle, then you got to add a crutch.
