Ooh, interesting idea... and really, really weird.

The quickest answer (aside from "don't do it" which is also the boring answer, bah) would be just to treat them as roughly equivalent; in that case, Ephemera is just Vitae by another name, harvested in a different manner. Given that said harvest only does bashing damage by default instead of lethal though (until you get to higher levels, hmmm), maybe it's more diffuse, and it's a two-to-one conversion (which incidentally keeps with the two resources' respective costs for healing as well, at least until the third dot comes...