I can't speak for all central vac systems but this is how mine powers on. When you insert the hose in, the end is metal and completes the circuit - see two wires near the top - black and red. Once the metal hose touches the little metal knob things it completes the circuit and powers it up.
In my house (3 years old) it was roughed in for central vac. All I needed to do was buy a Hoover central vac from Costco (around $500). It came with the wall inlets, floor pieces and of course the main unit. Without the wall inlets it was just a hole there, with a wire ready to be connected to the central vac inlet.
Here is what a similar central vac kit looks like:
http://www.costco.ca/Electrolux-Cent...100107035.html
With my system there is no power button, aside from one on the central vac unit itself which is in the garage