Vayne/Kalista which to pick and when?

So, I'm an adc main and my favorite champions and Vayne and Kalista. I just started playing ranked on Friday and im 7-1 so far, 5 of them Kalista games and none of them Vayne games. I was scared to pick Vayne because I know that she's not as strong right now, but I really love playing Vayne just as much as I love playing Kalista.

Would there ever be a reason to pick Vayne over Kalista? Obviously I know maybe in some cases a different pick might be better but assuming this was my only adc pool, (and that kog'maw is banned so i can't pick him) in what cases would it be better to pick Vayne or should I just keep spamming Kalista in my ranked games that I will be continuing to do?

(btw I got silver 5 in early season 5 then stopped playing ranked up until now so you can consider me low silver/high bronze level but I've played many games since)