No vacuum at all used on the HVAC controls of a GMT400 that is equipped with A/C. Even some of the factory service manuals still refer to vacuum lines so erroneous information is out there all over the place. All 3 actuators are electric, no cables, no vacuum.
The actuator for the vent positions can be problematic, or to be more specific, the linkages on it. (It's the only one with linkages.) Look above the driver's side of the transmission hump and you can see this actuator. The linkages are 2 sets of metal strips that slide against each other, with a spring in the middle.
The strips are lubricated with a thin grease and over time it gums up. You can gently clean them by spraying with some WD40 and wipe them, work them carefully through their motions, lay some newspaper on the carpet to protect it. Re-lube with a little bit of white lithium grease.
Sometimes the push nut on the linkage can fall off, and the linkage can also fall apart.
The 1995-up controls are somewhat notorious for going bad in a way that prevents proper actuator function. Especially if the radio has been out and the antenna lead left to dangle; it will short out the exposed circuit board on those controls.
Worst case scenario, and last one to worry yourself with, is it seems more likely on the 1995-up trucks for some reason that the flap itself can break right at the end where it pivots and the linkage attaches.
Richard