A - Vayne's passive is fine. While yes I do get where you are coming from when you say that you don't want a ADC to charge head first into a battle , it is a powerful passive , especially on Vayne because ...
B - Vayne is a late game team fight monster and janitor , if played correctly. Everyone has had a game that they crushed the enemy vayne to a pulp , but she still ends the game with a good KDA and winning the late game team fight. Small reminder to help you see why , her passive is tripled when she ultis. Meaning that she can ulti and use Q to go into stealth and get away from a situation , kiting back essentially speaking . Then when she feels like she can do her job , she will head into the heat of battle with her bonus 90 movespeed , kiting and smashing your head. Everyone has seen montages of Vaynes doing crazy stupidly awesome shit , and a lot of that is possible by the kite and chase potential provided by her passive