This question doesn't seem to be answered in the series. Vincent just says he's been watching her for 9 years - after her mothers death, and after he killed Cat's mother's assassins, and he always wanted to meet her; no doubt why he came out when Cat went poking around his hideout.
I usually do not watch this kind of show, and it's predictable to an extent, but I keep watching and wonder if the show will start filming in Paris and later move on to other parts of the world. I very much enjoyed the development of the story, especially the tension between Vince and Cat, and probably watched the ending of S1E14 about 3 times.