I'm pretty sure that they state she is a late game hyper carry tank buster champion, though if we are going based off her fantasy lore well the vayne in lore is vastly different then the vayne we fight against.
She cannot be considered a duelist because generally speaking duelists tend to excel in 1v1 situations but are rather lackluster in teamfights by comparison, and even those duelists who are both considered hypercarrys + decent teamfighters have at least one glaring weakness to make up for it which is being melee.
heck take one of the premier duelist hypercarrys people often think of
he has decent sized CD's early game, punishes you heavily if your playing him and misuse or use your duelist abilitys too early or late and does not really hit carry potential until his 4th item, or third item if he has more then 4 levels on his enemy, his items are also rather pricy and while he has solid teamfight potential you do not often see that potential due too his melee range, and in this case his best option is still forcing 1v1's or if he is fed 2v1's by split pushing.
Now compare that to vayne who has excellent teamfights, excellent solo duel power and often spikes to hyper carry status after finishing 1 maybe 2 core items.
ADC should be weak to assassins in general, especially if your a ranged hyper carry duelist that builds all damage unlike your melee counterparts who must build hp and some resists to do their jobs, now some adc should be able to deal with/survive assassins if they play right such as 


, just not vayne because she gives up nothing in her kit to allow for that, where as all those adc have costs involved for misplaying vs an assassin or diver.
I don't hate vayne as a champion i just think she has far too many strengths and far too few weaknesses right now due to a combination of kit and items, shes not alone in this problem as draven, twitch also suffer these issues. Supports also factor into this but only due to how strong and ADC's defensive while being super offensive items are.