Don't pick Vayne, just never. Low-mid elo knows that she's most scary ADC, one of the easiest champs to get penta and the one who can carry alone. BUT! She is high skill champion and requires: great positioning, being able to survive early game, gold (farming skills/few kills), sick kiting mechanics, brain and team for you to peel if you're focused. 90% times those bronze guys don't have any of the following abilities. Never pick her unless you're confident of your skill - hit plat and you can start think about her. Otherwise - NO!
Doesn't Varus require more peeling than Vayne? I mean, Vayne has more mobility, a knockback, and she can normally kill the person diving her if its a bruiser. And since Varus doesn't have any escapes, wouldn't he require better positioning?