Longtime no post for me. Good to hear you are still at it with your SVB.
The vacuum "pod" should be under the front cowling (to either side) and if my memory is correct, the controls should be in the fuse compartment (driver's side front compartment)
Check the size and quality of the hose too as it "should" be ribbed vacuum hose which is very pliable. If you have to replace it, go with silicone as it will outlast you. If your local AP store can't get it, it is a little pricier but you can get it at a local BMW dealership. Look for a mid-80's 325 (3-series) in their parts fiche and you should be able to find it.
'98 Gulfstream Sun Voyager Bus - Platinum with 275HP/880Ft/# Cummins and Allison 3060