The vac pipe is to do with DV as you know.

The turbo can spin faster and still hold the same boost if a DV is working like an overflow. Without DV the air has no where to go and is ultimately controlled by boost limiter, which will top out quick with all that air backing up in the inlet. All the DV needs to do is overflow a few PSI above the normal max boost pressure.

That way it wont leak under full boost, and only opens over it to allow turbo to spin as fast as possilbe while the butterfly is closed/closing for gearchange.