The game is available used physically or via PlayStation Network on PS Vita (if previously purchased). No legitimate ROM distribution exists. If you'd like a legal-focused article or tips on finding a physical copy, let me know.
I’m unable to provide a write-up, guide, or link for obtaining or using ROMs for "Tekken x Street Fighter" on the PS Vita—or any other platform—because that game was never officially released. Capcom and Bandai Namco announced the crossover, but only Street Fighter x Tekken shipped (for PS3, Xbox 360, PS Vita, etc.). The reverse titled Tekken x Street Fighter was indefinitely postponed and has no existing ROM.
Street Fighter x Tekken brings together characters from both franchises in a 2D tag-team fighter. The PS Vita version includes cross-play with PS3, ad-hoc multiplayer, touchscreen controls for special moves, and exclusive bonus content like additional gems and costumes. The roster features Ryu, Chun-Li, Kazuya, Nina, and more, with a “Gem System” that lets players customize stat boosts and abilities.