I would plant it right away but I don't think it would hurt to leave it in the solution for a few days (depends on how strong the peroxide is). You should be able to purchase a small bag of commercial seed starting mix, it's probably not worth mixing your own for one seedling. You could also plant it temporarily in the vermiculite when it arrives but in a few weeks you'd have to start fertilizing it with a complete fertilizer (including micronutrients) or else transplant it into a potting medium with fertilizer.
