There isn't much to the VCV. Just a chamber, an arm, and a rubber flapper. The most common failure is the rubber flapper coming apart and clogging the vacuum inlet. They can be (CAREFULLY) taken apart and a new rubber piece installed in the flapper. There isn't really a bench test since they're a controlled leak, they leak more volume than a hand vacuum pump can supply.

__________________
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]