I rarely build qss(Quicksilver sash) and almost never take cleanse. Which leads me to ask when Vi targets you with her ultimate (Assault and Battery) when should you use qss or cleanse to avoid the knockup and damage from her ultimate?
QSS or cleanse have no effect on this kind of spell.
To understand this, think of it this way, Vi's ultimate is an instant cast, once she used it you cannot cleanse it because it is not a DoT (Damage over Time) nor an effect, therefore it is NOT a debuff, however what you can do is preventing it from starting with a spell shield like a banshee's veil or Sivir's E.
Cleanse can work on things like Mordekaiser's ultimate or Vladimir because those are DoTs or effects, so you can remove the effect.
The only ways to preventing Vi from knocking you up with her ult are basically Spellshields, CC Immunity and Killing her before she reaches you. Tenacity and Cleanse have absolutely no effect on reducing the ability so the only way to "reduce" it is by becomming immune to it.
The spellshield is available to every champion through Banshees Veil while Sivir and Nocturne have one as a skill on a rather low cooldown.
CC Immunity is rather rare and can only be obtained through either a Morgana shield (castable on every champion) or for Olaf himself through his ultimate. Spells Like the ultimate of Vi or Malphite will also give you CC Immunity for the duration. After the spell animation is finished you will still be CCed.
According to the Wiki Knockups are somehow interruptable by using a cleanse effect and dashing but I have never actually seen this in 5 years of playing LoL.
Through the CC immunity that Vi gains while casting her Ultimate, you cannot interrupt her however her Q is interruptable and of course if you land a stun on her before she casts it, she won't be able to do so.
Invulnerability will work aswell so things like Zhonyas, Fizz E and Shaco R also interrupt her channel.