The vac res is in the passenger side fender and requires removal of the lower front skirting behind the front wheel. (with the double vents) It is not easy. It seems that you have a complete failure as your system is in the "default" mode, this happens at start up until enough vac builds up to move the flaps. It is also defaulted to the recirc as well, so that means no vacuum is recirc and air through defrost, and side vents. The problem you seem to have is no vacuum.
http://www.mbca.org/northtexas/vacuum_pneumatic.htm
On this diagram you will see the vacuum line that comes from the manifold to the vac distributor, then to the resevoir, I would ensure this line is ok (do you have any engine idle problems?) Then attach a manual vacuum pump to this line and pump up the system. See if it holds, if not you have a leak, let me know the results and we can go from there. Good luck!
Brian