I just realized there may be vacuum leaks involved and just ignoring my non-working cruise control might be yet another stupid thing I do. (88GT 2.8) How important is it to 1) get fixed, or 2) plug holes?

Just plug the hoses and ignore the system.

There's only one vacuum feed for the entire system. It tee's off of one of the two metal lines that run along the trunk wall going to the evap canister. The tee is vertical and is really close to where the driver's side strut tower meets the trunk wall. There's a rubber hose that connects from the tee to the solenoid control valve. As long as the solenoid valve works, ie it remains closed and prevents the vacuum from being applied to the rest of your leaky cruise control system, you don't need to do anything. Of course if you accidentally request cruise, then you'll lean out your fuel mixture due to the extra air that the leaky cruise system will allow into the plenum. If you want to eliminate that possibility or if your solenoid valve doesn't work, then the best option is to unplug the cruise vacuum at the tee fitting and stick a short length of plugged vacuum hose on the metal tee stub.

Sounds like a good idea to cap. There have been several times I'd find the cruise slider switch was "on". Thanks for the tip.