Some of the Singers are
ON/OFF and Kanon Wakeshima [japanese singers]
"Rondo" by ON/OFF
Kanon Wakeshima
The song is called 'Rondo' by ON/OFF. or is it MWHAHAHAA ok (cough cough random...)
Season one of vampire knight: opening: Futatsu no kodou to akai tsumi (by ON/OFF) ending: Still Doll (by Kanon wakeshima) Season 2 of vampire knight (guilty): opening: Rinne Rondo (by ON/OFF) ending: Suna no oshiro (by Kanon Wakeshima)
Yuki Cross's theme song from Vampire Knight is just called "Yuki Cross's theme". It's the third song down listed in Vampire Knight: Original Soundtrack.
coppret
If you are talking about Your Love is a Song, then the person who sings it is Switchfoot. I'm not sure if this is the song you're talking about though.
"Holiday" by Vampire Weekend
Try the theme songs for Vampire Knight and Vampire Knight Guilty, also Forbidden Act. Even though the first four are in Japanese you can find the lyrics with the song on youtube, they are all about death and unforgiven sins, and forbidden love, and heartbreak and sorrow and life sucking... so yeah try them,
Still Doll by Kanon Wakeshima
Holiday - Vampire Weekend
Vampire Weekend. The song is called "Holiday" from their new album "Contra"