You don't need a campy derailleur for it to shift properly. The issue will be the shifter uses a certain amount of cable pull per cog. The problem will be the spacing of the cassette will have a different spacing than the Campy cassette.

There is a product called J-tek shiftmate that compensates for cable pull and allows you to mix campy and shimano. I have not tried it or found too many reviews.

http://www.peterwhitecycles.com/Shiftmate.asp