Vi, similar to Varus, can begin to charge her Q, and then use the recall button to instantly interrupt and cancel the effect. However, unlike Varus, Vi's ability instantly goes off cooldown, and allows her to begin charging another ability afterwards.
This presents us with a very difficult situation in lane: A Vi can constantly charge her Q, cancel it with a recall, and then begin to charge it again without consuming any mana. While this in itself isn't necessarily powerful, it allows a Vi player to create bad situations for their opponent in lane without any real counter play.
The player on the receiving end of this has 3 options:
1. Run out of range, out of the range of the Vi Q.
2. Run randomly attempting to dodge/juke the Vi Q.
3. Proceed as normally, playing the lane as if nothing has changed.
But here's the issues which the above mentioned counters:
1. You effectively zone yourself out of experience, and last hits. Vi never uses her Q, and can continue to zone you this way forever.
2. Probably the best solution as the three, however you will likely miss CS due to the random walking.
3. You get hit by Vi, taking a fully charged combo. She damages you based on % hp, and gains a shield so you cannot effectively trade in return. Don't forget that Q also CC's you when it hits.
After a few minutes of testing, this only works if you spec into the improved recall mastery in utility tree. I will upload a video showing this in a few moments.
TLDR: Vi can spam Q and cancel with recall in lane and give enemy laner little to no counter play options. Must have improved recall mastery.
Edit: Video here showing how the bug works http://www.twitch.tv/therealzerve/b/357757499.
Edit2: New video explaining how this is problematic to play against as a top laner http://www.twitch.tv/therealzerve/b/357824430.
Oh and might as well give some clarity on Q's intended functionality.
When you use Vault Breaker three things can happen:
A) It will be used and you will dash forward
B) You will choose not to use it and let it expire
C) It will be canceled by some type of CC.
What should happen in case A is obvious. Skill goes on full cooldown and costs mana. Hopefully you ended up clocking someone.
In case B the skill should ALSO go on full cooldown and not refund any mana. We fixed it for normal recall but forgot to account for the improved recall mastery.
In case C it should refund the Mana and CD.