I really don't understand how people lose operating system discs. I mean, I can 'maybe' understand losing a game disc or two, though that's never happened to me, but you don't hear of people losing their keyboard, monitor, speakers or other components to a PC. The operating system is about as important as you can get when it comes to components of a PC.
OT, if you have a legit key for a retail copy of Vista, then contact Microsoft and request an replacement disc. It still should only cost you a fraction of what you paid. If your copy was OEM, you'd have to purchase a new copy anyway, so I would suggest a retail copy. That way, you can upgrade hardware til your heart's content, and always be able to reinstall the OS with no issues.
Oh, and keep the discs in a safe place.