Check 2Phast's site, the vacuum adjust is well documented there. On the W201's (2Phast has Hellas and I have Bosch) the aduster is on both left and right sides.
I don't have the part numbers for the W123, but for the W201 it's
Adjuster Switch: Bosch 0307 650100 041 081 or
MB 000 800 0473
Switch Cover (Rosette) A 201 689 01 80
For W124 Owners, I found this site:
Euro Light Adjuster for W124