Vi's Ultimate being Interrupted/Cancelled

I posted this about a week ago but it went unacknowledged. This is important so I'm posting it again.

Recently, Vayne's Condemn (the ability that knocks you back) has interrupted/stopped/cancelled my Vi ultimate in two different games. This concerns me because in the description it says "While charging, Vi cannot be stopped" and on the LoL Wiki it says "Vi becomes immune to CC".

I'm 99% sure this problem isn't related to lag because I play at 49ms (virtually no lag) and I've been stopped at both the beginning of the ult and at the end (just before I reached Vayne).

Is this a bug? I really don't know what else it could be unless Vayne's Condemn is a special form of CC that works against anything.