my 1987 W124 230e. Was followed by awful, loud vibration of serpentine belt, belt tensioner shaking.

The cause was a nut and bolt that had come loose from where the hydraulic damper strut is attached to the belt tensioner. Removed shroud and fan, replaced nut and bolt, used Loctite, all good again. The thread wasn't stripped, thankfully.

I'd say try to reattach the strut. Your belt may be OK, but if the belt tensioner works loose it's moderately ugly. Mine was driveable (in limp home mode). Cheers.