Yes it is a tax, a vehicle tax. Just like petrol 'duty', alcohol 'duty', tobacco 'duty' are all taxes.
I'd happily pay 'road tax' if it was ring-fenced to pay for the upkeep of the roads but the money raised from V.E.D. isn't.
We will be paying 'road tax' before long when they fit black-boxes to all our cars/bikes and charge us £ per mile, with premium rates for 'peak times' which will no doubt be 7am-11am and 3pm-7pm, as well as sell the data to whoever wants it - no doubt that won't pay for the roads either
