The Monoprice shock/vibration sensor is absolutely worthless for this purpose. I did put it on a spring (surprisingly enough just like @john.hart3706) with some double sided tap and it did work a little better. The reason it doesn't dampen always is because eventually you have basically constructive energy where the spring is moving slightly in a direction and the it gets a boost in the same direction from the dryer.
At any rate, it wasn't reliable because the Monoprice sensor, even set to most sensitive and on a spring, would not trigger and stay triggered.
What I eventually did was got these (one for my dryer and one for my oven). These essentially support an external dry contact sensor aka a sensor that doesn't require voltage but on it's own either completes or interrupts the circuit.
At that point you just have to get a dry contact amp sensor. (My dryer is electric. I'm assuming yours is too or you should just get a power metering 110V plug and watch the power spike to know when it's on.)
*edit: By the way, dry contact amp sensors are called current transformers or CTs. There are two types: split core and solid core. Split core means that they act like clamps and you don't have to disconnect the wires because you can clamp them on around a wire. Solid core mean that you have to disconnect the wire so that you can slide them on from one of the wire ends.
You have to choose the amperage at which you want them to trip or the "trip point." I choose .5 amp CTs because (from looking at my HEM) I could see that the dryer always pulled an amp or more when it was running. This was one of the cheapest I found so I went with them. They don't come with contact wires, just the CTs. I have wire laying around.
