I think cello is a bit low to substitute for most vocalists, although it does have a very "human" sound to it. One thing you could try is downloading this:

http://hypermammut.sourceforge.net/paulstretch/

Find a vocal snippet, stretch it out by 100 times or so, and now you have a pure voice "texture" that isn't readily identifiable. I've used it in a few tracks and it can actually make a neat "pad" type sound as well.

Well I know this is obvious, but it will all depend on the vocals. If they're already written, then yes, if they're not, it'll be much more difficult.

A Sax, Trumpet, or Flute.

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