so whats stopping vayne and kaslita form being fotm or must pick/ban champs?

Are you inclining that they aren't?

No one bans ADCs unless they are absolutely ludicrous. And, with good reason too. Most ADCs play very similarly to every other ADC. They are like Legos, for the most part: You can take out a red one and put in a green one, and the team will barely be able to tell the difference.

Champs for other roles , however, are noticeably different.