The Medway Handyman wrote:

Can you buy said gizmo to fit to an existing vac?

I've not seen a task-specific gizmo, but how about the "intelligent"
power strips that are sold for turning off all the crap round your PC
when you turn off the computer itself? They have one socket that's
always on, and others that are on only when at least 100watts or so is
being drawn from the first one.

Pete