Use a Campy 8 or early 9-speed era rear derailleur and an unmodified Shimano 10-speed cassette.

Veloce, Daytona, Athena, whatever. Should cost about $50 on Fleabay.

The cable pull of the Campy 10 shifters, mated with the 1.43 actuation ratio of the older Campy derailleur will perfectly match the Shimano 10-speed cassette spacing.

The math works, and I've been running this combo on multiple bikes for many years. Flawless shifting.

Another combo that works is (the later) 9-speed Ergopower shifters mated with the older Campy (year 1993-2000) rear derailleurs. Shifts flawlessly over a Shimano 9-speed cassette. This is what I'm riding today. Perfect shifting.