From a jazz perspective, the #9 is just a color tone. So the C7#9 functions as a C7 chord.

If you're looking for something that leads away from the C7, the most obvious chord would be a F, as that's your typical circle of 5ths progression.

If you want something to lead back, the circle of 5ths would suggest a G7 chord of some flavor.

That having been said, you don't really have a tonality in mind (you don't know what the function of the C7#9 is), or any particular plan other than a vamp. So it's a bit hard to suggest what direction you should take this one-chord song.

Does it really matter what chord you choose, as long as you move away from the C7#9 and then back to it?

-- David Cuny
My virtual singer development blog

Vocal control, you say. Never heard of it. Is that some kind of ProTools thing?