I think I made a significant find for this conversion. There's a company called EV parts that offers a vacuum switch designed for this type of application. I found them online and they're EV as in Electric Vehicle Parts. Inc. - they specialize in bits for EV builders. The woman on the phone said that they have these built with the right parameters for automotive use. You'll need to run the pump thru a relay but that's not a big deal.

EDIT: I just double checked and the vacuum setting is adjustable but comes from the factory at 18-22"Hg. As I understand it, this means that the pump will switch on at 18" and off at 22". I'll crank mine down a little as I don't need that much just for shut-off.

The bottom line is you fork over about $40 (with shipping) and you get a device that will switch the pump off and on as needed to maintain a pre-set vacuum level.

Mine's on the way

Dan