The under hood vacuum pump, to my knowledge, is like all vacuum pumps. It sucks down to the appropriate vacuum and quits. Now, if there is a vacuum leak, it will run continuously when you switch your dash control to a function where there's a leak in the vacuum hose. If you have several leaks, well, you can imagine.
Basic operation of the dash controls requires a bit of vacuum to actuate a vacuum device. Once it's actuated, the vacuum should hold it in position...unless there's a leak.
So what you should be looking for, are leaks in the hoses. On my rig, and my friends rig, there's a vacuum actuator on the outside firewall under the hood where the elements had caused dry rot in the vacuum hose. It was easy to find because it was loose and just hanging there in both our cases. Fixing that with a short piece of 1/8" vacuum hose from Autozone fixed both problems.