To use her really well in a teamfight is actually kinda hard.
You can use her in a teamfight with no skill at all, but you won't be anywhere near peak performance. Plus you can get zoned, and being zoned means no damage whatsoever because she has no damage at all outside her AA range; she provides no real utility unless she stuns, but most players save that for when they're jumped on, limiting her impact in a teamfight, especially when she's behind; she only has things relating to surviving for herself or doing damage.
Varus has a long-range poke, a medium range poke (with GW), both of which can extend past his AA range, and snare that can spread (doesn't always happen, but happens enough to warrant mention), and has decent all-in with his W passive on ability hit. All of these mean that zoning him doesn't mean he's useless, and he can provide utility to his team much more reliably than Vayne. This tends to make someone a bit easier since they don't have only 1 source of damage worth mentioning and a situational CC/peel.
So yes, she's usually a good deal harder than Varus. Just because assassins can jump on him doesn't make him harder than another champion just because they can deal with it - they have issues in other departments. Plus if you don't have split-second reaction speed, Vayne being able to reply will mean nothing if you can't do it yourself.