Vi is a very strong jungler and fighter and her ult allows to shut down an enemy adc very quickly before the team fight breaks out. After going all in your passive and defence will allow your team to follow up to finish the others.
The problem with Vi is that she doesn't have any slows or stuns to stay locked when chasing, although her q is powerful to keep the chase on but after a few punches you are behind an enemy again.
The best thing is that Vi ult won't stop until it hits her target. Which normally cause players to panic and waste a mobility escape or flash.