Fitting Shimano cassette to an old mavic wheel
Campy and Shimano 9-speed cassette spacing are too different for the mismatch to work very well. Campy is 4.55mm cog-to-cog and Shimano is 4.35mm. It is true that it can be made to kinda sorta work, i.e. in the stand when things are clean and new, but it's finicky in real world conditions.

It seems like the Veloce 9 28t cassettes haven't been available for a while. Miche is making a Campy-compatible 9-speed 13-28 which is available though. Your simplest route by far, especially if the rear wheel is still good, is almost certainly to get that cassette and then if that isn't enough, switch to compact cranks in front.

The other path is get a new Shimano-compatible rear wheel, or build a new hub into your current rim if you really want, plus a Shimano 9 cassette that has individual cogs, and then conversion spacers for it that switch the spacing to Campy. These are the good way of doing this kind of hack. 9-speed ones have been available but I'm not sure if anyone is making them new anymore, but if not they can probably still be found. 

You probably have a Campagnolo (as in made by Campy) hub now, and no replacement freehub will be available for it. At this point it's almost certainly the same deal if it happens to be a third-party Campy-compatible hub, although it is true that sometimes swapping freehub bodies on those with another type is technically possible.

(It sounds like you've got a wheelset with Mavic rims and someone else's hubs, as I don't believe there was ever an Open CD prefab set. This bike is also from before the days of named prefab sets. If I'm mistaken and the hubs are also Mavic, then there very well may be alternate freehubs out there for them, as Mavic road wheels have typically emphasized being fairly easy to switch.)