It looks like those UMDs you bought for your PlayStation Portable will be completely useless if you decide to upgrade to a PS Vita.
The PS Vita will play PSP games but they'll have to be of the variety that you buy and download from the PlayStation Store. Sony announced what it called the "UMD Passport" program for Japanese consumers, but players in other regions will not have access to that program. Sony decided that we didn't want or need it. The UMD Passport program would allow gamers to register the games they already had on the PSP exclusive format and then download them onto their spiffy new Vita devices.
Sony has not indicated why it won't roll out UMD Passport outside of Japan, but it certainly gives consumers an incentive to hold onto the aging Sony portable platform. In fact it may prove to depress sales of PS Vita to those customers who have invested a lot of money into UMD-based games. We'll see.