It has interesting possibilities, but the challenge would be writing it so those two could plausiby feel an attraction while remaining true to their characters. I don't think it could work out, given Tifa's love for Cloud and Vincent's lonely vigil for Lucrecia. You'd have to write it so that Cloud and Lucrecia were out of the picture, if only temporarily. If you kept the story setting as is, I could imagine them forming a platonic bond. Tifa seems very caring and nuturing to basket cases. And I feel Vincent does have a dark romantic side to him.