It would still cost them money to make a Vita-specific version. It isn't like they can copy-paste the PS3 code to a Vita cartridge.
how much money, do you know? I can't imagine it is too difficult or costly, especially since the HD versions are already made.
Some of these HD versions tend to have new issues even on PS3, performance-wise mainly. I really don't think they could all run on Vita without the expertise that most of the handheld or bottom tier developers they assign to these projects lack.