Hi!
Whatever the billing, I find some interesting logic in this character. She caught Vince's attention, immediately tried to use him to get her job done, then, with whatever intent, merrily slept with him, and was ruthlessly used in her turn.
When she asked whether he had a cat, he knew already it was not a cat, and just sent her out there to win time and possibly split the hitmen. The scene with her throat being scratched also showed just how little he valued her, and what kind of gambler he was.
I don't think Grace stayed with him after that incident, so what was the point in keeping her around any longer? Vince's inherited charm was confirmed, his touchness revealed, so Grace's role was over.
Best regards.
Alexander