I suggest Miles Davis' Kind of Blue if you want something very laid back (If you going to hit up the p2p networks, I would look Blue in Green and Flamenco Sketches.)
I'm a big fan of Coltrane, so I would suggest Cresent. Much of his music is ultra intense (as a matter of fact, Love Supreme was done in the same year as Cresent), but Cresent is real change of pace.
Thelonius Monk had a nice album on Columbia records in which he plays mostly standards on his own, but I can't remember the name at the moment--I'll have to dig it out at home. But that would also be a nice choice because it's also ballard heavy.
edit: For some reason I missed the part about vocalists, but these are good choices if your just looking for something mellow.