Ingrid Michaelson
Regina Spektor
Bright Eyes
With your metal tastes they may be a bit too folk/pop for you but if you're looking for music driven by beautiful vocals then they should be up your alley.
Ulver
Similarly to you, I listened pretty much exclusively to death metal, grindcore etc. for years, one of the bands that really helped me expand my musical pallet was Ulver. I was surprised to find a band with such a diverse discography driven by such beautiful vocals. They began as an old-school black metal band, got really deep into the minimalist electronic sounds, went heavier into the electronics with some jazz influences....
Just download/purchase "Blood Inside" and you'll see what I mean.