Idle vacuum should be roughly 12", measured both at the output of the VCV and at the modulator. The VCV itself may have a clog in it, they're basically a spring-loaded flapper with a rubber disk on the end pressing against a metal protrusion. Over time they can wear away and that metal protrusion can get chunks of rubber in it and clog it up.

Make sure the vacuum hoses are on the VCV correctly. It matters. Vacuum supply should be in the center port, BUFO should connect to the off-center port.

Depending on your setup, there may also be a switchover valve to tie into the boost sense line. If it's leaking by, your vacuum will be low. Measure vacuum output directly at the VCV, that will tell you if you have a problem with the VCV or with something downstream of it.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]