The vacuum reservoir is under the battery tray. Make sure the supply line from the intake manifold is all good, not cracked or broken anywhere. It runs from the intake, to rear of engine, across the firewall, tee's to the HVAC inside, keeps going to another tee right at the cruise servo on pass inner fender and then goes forward to under the battery tray. There should also be a vacuum check valve where it connects to the manifold.
If the HVAC is stuck on DEF and nothing else the line is probably broken in half or cracked/split up real bad. If the HVAC works but goes to DEF under acceleration then there is probably a small crack in the line or the check valve is faulty.