I have 3 "normal" returns (just straight pipe through the plaster) and another "return" for the automatic vac./cleaner. I have rubber plugs that fit the 3, but the vac return has a twist lock type connection to it, and doesn't appear that it would accept any of the plugs I have. Do I treat it like the main drain and just use the valves to cut it off once I'm blowing air out or should I have some sort of cap/plug that fits it?

TIA

Welcome to TFP!!

The twistlock connector should unscrew from the wall, allowing you to use a threaded plug in the port

If you're still having trouble with this, let us know the model of your cleaner and we'll help you figure out the best way to winterize this line