Eventually it can fail, however that usally is when B2 band breaks or the pin falls out.
Af for checking the vacuum, there are several things to check and several vacuum components involved in the system.
The first thing is to "T" a vacuum guage to the black line that goes to the transmission. At idle depending on the model, about 14" of vacuum and bacically upon full throttle should drop off to or near zero (basic check only).
Mercedes-Benz Master Guild - 2 years
Shop Foreman - 20 + years - dealer.
Part owner - MBZ Transmission remanufacture