I wouldn't add more vermiculite, it would make the crowns be deeper. It doesn't matter if the roots touch the bottom, they'll just run along where they can.
Quite possibly the MG didn't do any harm but the logic is that if you water with something with salts in it like a fertilizer it may draw water out of the roots if it is concentrated enough, or at least make it harder for the roots to take up the water (osmosis).
