Her low range
TL;DR at the bottom.
I looked up the exact ranges of every ADC, and isn't 550 average? As in, Vayne has average range?
(Stats taken from wiki)
Kog, Kindred, Sivir, Lucian, 500. 2 people with range extenders and 2 lane bullies. Also Sivir gets a speed boost and Lucian has a dash that can reduce it's cooldown.
Xayah, Jinx ( Fishbones gives extended range) 525. Xayah is supposed to be a lane bully (I think)
Twitch, Ezreal (Lane bully), Corki (mid laner at this point), Kalista (lane bully), Miss Fortune (Lane bully), Jhin (lane bully(?)), Draven (Lane bully), Vayne 550.
Varus 575. Probably supposed to be a lane bully, but everyone I see goes
first item and starts going all in with ult, little poke.
Ashe 600 (I swear this is an error unless they buffed her from 550, as I recall fighting a Vayne at max range during the summer).
Caitlyn 650. Lane bully.
Tristana 550-669. I have no idea what she is, other than an assassin like Twitch and Vayne (Why are the hyper carries also assassins ( supposed early game monsters)? I don't know, but they are according to the client's champion sorter)
Unless in-game says otherwise (as I don't own vayne, so I can't actually check), her range is average.
Vayne has the same range as most ADC's (says the wiki), and more have lower range than her, compared to those who have more.