Your silver box behind the glove box is likely bad. If the vents open at all under any circumstance, you likely don't have vacuum issues. The other possibility is bad vacuum pods on the recirculate flap (feel for air leakage through the defrost vent).

__________________
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]