If you're going to go to all the trouble of making a vacuum switch, why not go all out and make it a bimetallic strip? Then you have a circuit breaker at a certain high temp, but never let the bowl cool. This way you always have perfect smoke ready. If you just have a vacuum switch your coal is only on while you're pulling, so it'll take a lot of pulling to heat your bowl up to smoke. If you walk away for a few minutes you're left with a cold bowl. I guess it would be more safe, but with a bimetallic switch in the right place your smoke is always ready and you literally can't burn a bowl.