And you still have the same mass of vapor moving at the same speed. At those speeds the state of the matter doesn't real make any difference. You will still impart the same kinetic energy and momentum to the target (earth) maybe spread out over a larger area, but this wouldn't necessarily be a good thing.
Yes, but the mass of vapor/plasma/tiny particles would probably be handled by the upper atmosphere then, akin to solar flares. That would probably factor into the distance calculation.
I forget the author off hand, but there is a great series, Star Marines, where we strike a heavy blow against a ridiculous foe (essentially responsible for the Fermi paradox in our Galaxy) by transiting an older sub light starship through a gate into a system containing one of their major hubs. The cargo modules contain 50,000 tons of sand mined from Mars which is dispersed en route after the transit. The author did a great job describing the theoretical results.
That series was one of the best Sci Fi works I’d read in a while.