I'm sure there is a spec somewhere (VESA or something) but here are a couple of references. Power is watts = volts * amps. Even though there was a +5V pin on a VGA connector, it may not be suitable ...
Do you also hate it when you drop 800 bucks on a graphics card, and spaghetti of wires to the power connectors remain visible with that vertical mount,. Well here is an idea, a small converter PCB ...