Let's see...they already have something like that...it's called the...get this... "GAS TAX!"
That is the "fairest" way of finding out how much you "drive" because gas is a fixed commodity. In fact, it is fairer than a VMT, because if you drive an SUV or GOP-type car like a V8, then you pay more per mile, while your Democrat-type car, the Yugo and the Prius, you pay less. So we ALREADY have a tax in place!
SO...what does the government REALLY WANT?
The government wants an itemized location record of EVERY TRIP YOU MAKE. In addition, such records may even be able to use for retroactive speeding tickets, or they can use it in any number of ways to screw you.
That's why VMT will never happen, even in Socialist California.