She can 1v1 some assassins, but she can 1v1 nearly all tanks. So a shift from tanks to assassins makes her slightly weaker.
I feel like if it's OK for an ADC to be a strong duelist, then her weak point ought to be lower teamfight presence compared to someone who can be part of a team shredding wombo combo (MF, Twitch, maybe Lucian), have mage-like poke (Kog, Corki), or contribute strong team utility (Ashe, Sivir, Varus).
Or other ADCs could just tear up towers really damn fast (Tristana, Jinx).
And it's certainly true that Vayne doesn't really do any of those things -- her entire kit is single target and she's discouraged from buying Runaan's (because it doesn't work with her W) and Shiv (because the high power of her W rewards AS more than crit).
But neither do her opponents if she's already killed them.