Turn vacuum on when another load (eg. drill/saw/etc.) is active
You could buy an non-invasive current sensor, a 5V 10A relay or two relays switched on together to drive 20A and an Arduino Pro Micro.

With that "setup" you can easily hook up the relay and the current sensor to the Arduino and program it to turn the relay on, if more than xx current is flowing through the sensor.

Because the sensor is non-invasive, it means you can put it on the tools power cord without cutting the AC wires (or doing other dangerous things) and it'll work just fine.

How you can read the sensor with the Arduino you can read here and turning the relay on is as easy as writing two lines of code (setup pin xx, set pin xx).

It'll cost you less than $20 (if you pick an Arduino clone), and it will work for ages, as the Arduino has practically no parts that can fail over time.

I made a image showing how it will be connected:


(To measure the current correctly, you should pass only one wire of the power cord to the non-invasive current sensor, not both. If you don't care about it and just put the entire cable through, it won't measure the current properly, as the energy is flowing in a loop, resulting in [near] zero.)

The Arduino Pro Micro needs a very very very tiny amount of energy (approx. ~10 mA), this means any super-cheap micro-USB wall adapter will work just fine. There are even instructions to solder out some components and including a library in it so that it doesn't consume more than some uA!

EDIT: Oh, and as you may have guessed, you can plug it into any USB power source to power it (PC/Notebook/Wall Adapter/Toaster?/...).