@johnwales
You do NOT get 12V from a car adapter. We've had a few users mistakenly try to hardwire 12V to their units, and have experienced the smell of burning parts every time.
No matter what the source (12V to 5V USB car adapter), USB port on a computer, 5V USB wall wart, it's ALL 5V, and ONLY 5V.
Differences DO exist in the current capability of various charging sources, and will impact charging time, or even the ability to charge depending upon the source and device. For example, if the device uses 500mA or more just to operate (and while I don't think his Via fits that category, some TT units do), then plugging into a source capable of only 500mA output (most computer ports, for example) won't provide enough current for charging with the device operating. With the unit off, a 500mA source off a computer will charge the device at just about half the rate of a 1A wall charger or cigarette lighter adapter.
@andrew
You've said "It gives no warning and has to be re-booteed". A reboot would seem to indicate that the battery is completely flat.
You've also said that "The tomtom is charging ok. It seems that when I have it plugged into the car charger and turn my ignition off, the tomtom sometimes turns off when I restart my car." Is your Via set up to shut down with loss of power for battery saving? Tell us how you have it configured to handle power-up and power-down.