I don't think the actual vacuum reservoir would be the problem. They are built out of some might tough plastic, mines nearly 29 years old, still looks brand new. The vacuum line going to the reservoir or the check valve may be bad. Could also be the early sign of a door control unit failure, a small tear in the diaphragm that sometimes seals itself.
My door locks were intermittent for a while until the right rear upper diaphragm finally failed completely.
Remember you have three units (if a 4 door) in the doors (driver's door has the master valve), a unit for the trunk lid and one for the fuel door. That is on top of any climate control lines or engine shut off lines your particular vehicle may have.
I have both a hand vacuum pump and a simple vacuum gauge. The hand vacuum pump will be indispensable in the diagnosis of your vacuum trouble. Pull a vacuum on each separate control unit, let it sit and see whether it leaks down or not over a given period of time. A bad leak will bleed off in a matter of seconds. Don't forget to pull a vacuum on the check valve too (that was my final vacuum loss point, only intermittent as the valve would sometime check sometimes not).
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.