You definitely have a vacuum leak. It's probably on the vacuum sub-harness that serves the HVAC and cruise control, with the reservoir at the end.
This specific behavior usually indicates that the outlet selection sub-harness on the back of the HVAC head unit has a leak. That sub-harness only pulls a few hg inches of vacuum so any localized leak will cause it stop working. Here's a pic of mine from when I had the same problem. All the colored lines go to different outlets and the head unit basically moves vacuum among them to open and close the different flappers. I took the clear plastic thing and cleaned it off then smeared a light coating of silicon dielectric grease to get a better seal. No problem since.
Basically you need to get yourself a vacuum gauge ($20ish at any auto parts store) and measure vacuum pressure at different places. Normal engine pressure is 15-20 hg inches at idle after fully warmed up. Test for pressure then disconnect and plug off the HVAC vacuum line (the nylon hose coming out the firewall by the A/C expansion valve) and see if you can isolate it.