Why can I not cleanse Vi ultimate?

Because it would be like cleansing Fiora Ult, Zed Ult, Fizz Fish, etc. You can't cleanse the debuff (her charging you) but you can Cleanse the CC.

The ONLY way to counter it is Yi Q, Fizz E, Zhonya's, and anything else that becomes untargetable.

Which is honestly pretty BS.

is your very situational friend...also if uses that to initiate, your team can easily punish her or burst down anyone who tries to follow up, its literally just a big single target gap closer that makes her unable to hurt anyone else aswell as you then takes a bit of your hp...

Although if the enemy has a that is a different story all together and I feel sorry for you.

If you wanna call bs, you should also look at the fact that she shreds your tank`s armor yet has an assassin like gap closer and mobility.