I'm thinking the healing from Vamp. Embrace will be sorely missed. Your Paladin will also take more damage if you spec him that far into Retribution, so overall I think its a lose/lose trade. IE. you trade in damage done, damage "resisted" and passive healing while getting ??? in return. Bigger "proper" heals and not having to shift out of shadow form?

Shadow Priests looks very tempting, I'm gonna have to start up a group one of these days :)