Neel, you make great points about the implications regarding a possible mileage tax. I think the general idea of a VMT would be to capture more revenue, even from electric vehicles, into the Highway Trust Fund. A VMT may also be better at targeting the behavior of drivers on the extreme end who drive the most.
However, I completely agree that there are many drawbacks; a VMT would require that governments track the movements of vehicles, and while there could be steps taken to track only aggregate distance and not location, such a power could easily be abused. The VMT tax would could also require installing new technology in all personal and commercial cars or would require every vehicle owner to periodically report distances driven, which would create a costly new bureaucracy and extensive audit system. Lastly, VMTs definitely have the potential to disincentivize purchases of less efficient vehicles by charging all drivers regardless of how eco-friendly their car is. As for the commuters, I could see the VMT theoretically as an incentive to use public transportation or carpool instead of driving, but given that many parts of the U.S. do not have adequate public transportation as an option, it would only increase financial burdens on commuters in practive. In all, a VMT seems intruiging at first glance but rather problematic to implement.