A lot of analysis and calculation has been done on the potential impact of large numbers of electric vehicles drawing on the grid.
As noted, most of the energy will come from coal-fired generators, but that’s been considered too.
One point: yes, most of the emission from a tailpipe is water vapor. That’s always been true. Carbon monoxide, sooty particulates, unburned hydrocarbons, and nitrogen oxides are never more than a few percent of the total emissions, even in old, badly tuned engines. But even a few percent was enough to create the smog that blanketed most cities. The emissions level has been reduced to a fraction of a percent, but it’s required heroic levels of technology to achieve that, and there are a lot more cars on the road.
Fossil-burning at a big generating station isn’t much more efficient than an internal combustion engine (especially after one allows for the energy cost of transmitting power to end uses). But it can be much cleaner: an IC engine burns its fuel in explosive bursts which leave nasty byproducts. It takes elaborate tuning and after-burn equipment to cope with them. Continuous even burning (to heat a boiler) is much easier to manage.
