SCE with the support of third party developers and publishers have been offering select UMD titles on PlayStation Store and will continue our effort to deliver these content in the form of game medium for PS Vita going forward.
However, Sony will need to clear all copyright / legal related issues and due to this not all software titles will become available on PS Vita. Sony will announce details for each title when ready.
If you have existing UMD games that you want to play on the PS Vita, Sony are looking into the possibility of porting these across like the PSPgo Rewards programe. There are no plans on adding a UMD Reader to the PS Vita, however.