At first, he was using her. But then he got to feel for her and sympathize with her. She got him to feel human again. To me, Vincent (being my least favorite character) is a sociopath and thus only loves selfishly. Everything he does is for Gil and he excuses all his wrongdoings on this. But I believe originally he would only care for Ada because Gil would want him to and because she's Oz's sister. However, he grows to respect her and when she forgives him, he finds a reason to live and stop beinf so self-centered and psychotic. I think it may have been platonic on Vincent's part as he said he could never stay with her.