I'm not a Campy expert but I do know that the later 9-speed rear derailleurs have a different throw ratio than the earlier ones, I think an early 9-speed RD would work but not a later one. Not sure but I think you would need a pre 2001 RD. Check with http://www.branfordbike.com.
Of course it you've already got the derailleur you could try it.

Al