I only see one reason that Vista may be better in a car environment. There are many reasons against it, most CarPC's simply aren't up to specs for the most part, and that right there is a really big reason
What is fantastic is the new hybrid standby/hibernate. If you put the computer in stand by, it also saves the info to the hard drive. If you do not lose power, the computer will start instantly. If you do lose power, it will just come out of hibernation.
I have never tested the system draw, power wise, of a computer in standby, but would imagine it is minimal enough to last at least a day. Considering a tiny laptop battery can run a computer normally for around 2 hours of use. A shutdown controller could then be set to say if x hours in standby, power it off, to prevent a dead battery in the car.