She seems to enjoy antagonising him a little, but it seems mutual too... I guess she is a little flirty, but I don't know about her being seriously attracted to him. However, at the end of the game, when (SPOILER)Vincent's burdens have lifted and he's beginning to move on, it's possible that he'd eventually lighten up enough to open his heart again. Maybe something truly could happen with him and Yuffie. However, Shelke is the same age as Yuffie, and has been through experiences as harrowing as what Vincent endured, if not worse. Now that she's out of Deepground, away from its daily Mako infusions, it's possible she'll catch up to her true age. She definitely seems drawn to Vincent, too...
So I'm going to predict that FFVII-2 is going to feature one hell of a bizarre love-triangle and/or catfight