Can I replace my Veloce 9 speed 53/39 with a Mirage 50/34 compact?

It'll work, and its not likely that you'll need to change your rear derailleur either. I have 3 bikes with compacts on the front and all of them are equipped with standard short-cage rear derailleurs, one Campy Record and two Shimano.

Campy makes a compact specific front derailleur, but your stock will probably shift it just fine if adjusted properly. If it doesn't, you can always upgrade the front derailleur later. Save your money for now and see how well it works.

P.S. to Tom: It's wary, not weary.

I wouldn't worry. Just slide your derailleur down the seat tube slightly, so it's only 2-3mm from the top of the teeth on the 50T.

I would however be wary of the rear derailleur. It might not be able to take the slack chain, as it need to accommodate the smaller 34T inner chainring - You *might* need a long-cage derailleur...

If your ft. derailleur in a clamp type you can slide it down to the right position. Since the tooth difference is only 1 between both setups you should be OK there and your sindle is good I say...

If you can move the derialleur you are good to go.