how do i defeat vi

You need someone on your team with hard CC if the enemy has a Vi in their comp. Vi's ult cannot be stopped, but once she is in the backline of your team, somebody should be applying slows/stuns to her while the rest of your team bursts down their carries. Once that's done, turn your attention to Vi and take her out.

Vi is strong, but you just have to use the proper tactics to take her down. She is a single-target champ (with a small amount of splash damage) and can be handled pretty well with CC.

If her ult CD is up, you don't have any ways of stopping her until after your carry is dead.