Recent (within the past 15 years or so) Campy double cranks have a 135 mm bolt circle so a 39T is the smallest chainring that will fit. The relatively rare Chorus and Record triple cranks had a separate 74 mm granny bolt circle that would take down to a 24T chainring.
Campy did make a 13x29 10-speed cassette that would be a big improvement over your 11x25.