I have a small power convertor that hooks to the battery of my truck, just in case. I haven't had a need to use it yet. I think it was from Tractor Supply. It just clips on like jumper cables and has an outlet on the side to plug in what you want to use.
I picked up a 1200W inverter at Sam's for something like $30, plugs right into the lighter outlet. - running the car while in use wouldnt be a bad idea, but you don't need a billion miles of cord. over 100' of 12 gauge would be a bad idea, and using 10 gauge to go farther wouldn't be cost effective - at all
To find the watt requirement for your vac, look on the nameplate, where it says amps, multiply that by 120 (volts), add 20% to be safe and that's answer in watts + 20% and the size inverter you should get. not too pricey and not too much cord to drag out.