I would think the vacuum leak afecting the shutoff is just afecting your climate control as well when cold out. Wherever it is. If vacuum is disconected from the climate system when off it may be an indicator of where the leak may be. For all I know the climate system could be under constant vacuum though.
You could quick test that by seeing how the car shuts down cold perhaps with the climate control off. Temporarily plugging a line might give separation as well. Really though your mity vac should quickly give some indication of where the leak or leaks are at cold temperatures. You unfortunatly have to do these tests out in the cold.