Never heard of a car with a 115v outlet (since cars are predominately 12v systems). Perhaps the car needs to be running in order to kick out that much juice through a converter.
It's the amount of amps the vacuum is pulling. The house system can handle it while your vehicles inverter is probably rated at a much lower amperage rating.
I don't know of any way to make the car's system 'more powerful' other than putting in a larger inverter.