Vi getting knocked back during ult.

I've been playing a lot of Vi recently and I've had this happen a few times. It seems that if a player casts a knock back of some kind right when she makes contact she gets knocked back and in a way stunned, as she's still locked into her knock-up animation. I haven't really had much concern until I lost a game because of it happening consistently against a Syndra. She was very good at landing it every time and every time it put me out of melee range when the cast was done, giving her a huge advantage. It even resulted in a tower taking most of my health as she managed to knock me back into tower range and during the animation I couldn't escape the aggro.

If this is intended, then please add a feature to cancel the cast so we can cut our losses if it fails. If this isn't intended please fix. It's very frustrating to lose a 2 minute CD for nothing early game, even worse when you die for it.