Hold your Q until the enemy's react. Usually they will flash and then thats the moment when you dash and kill them.

Use your Ultimate to avoid enemy CC.

You can chase the enemy by ulting just a second before he use his flash.

You can reset the atacks by using E. The combo usually is Q + AA+ E + AA + E + AA if you are lvl 3. When you are 6 you can try Q + AA + E + R + AA + E

The flash is very usefull on VI, if you flash while you are casting your Q you can konck up the enemy at a very large distance.

The flash also can redirect the Q. This is the most difficult mechanic. But if you practice in the practice mode a long time you can be the best Vi player

As Vi you have a lot of pathings in order to counter jungle.

1) Start red, jump the drake wall and steal enemy Blue, Gromp and Wolfs. Then the crab and gank. If you cant gank, do the rest of your camps an go blue.

2) Start red, jump the drake wall, steal Gromp and Wolfs. Then wait for the enemy jungler, steal the blue and kill him.

3) Start red, jump the drake wall, ward the blue bush and when the enemy jungle is doing his blue you gank top or mid, he wouldn't react at that.

4) Start Blue,jump the herald's wall and steal all the top side jungle. Then you can gank top going through the thribush or do the crab or back and do your jungle

5) Start Blue, jump the herald's wall, steal red and birds and then gank mid going through the birds wall using Q flash if the enemy is near his tower.

6) Start blue, jump the herald's wall, ward the space between red bush and the other bush placed in front of red bush, and then do your Gromp and Wolfs. When the jungler walks in the topside, you can gank mid or bot.

Vi is a powerfull and safe jungler who can stomp the early game and win at min 20.
To counter VI you need to control all her jungle stealing buffs and camps.
I recommend use Warwick or Lee Sin because she cant fight against thats picks, if she does it, she will die.
Vi cant retreat once she started the fight, so wait at her ultimate and then focus her by interrumping his Q when she is traing to scape.
The best option for a botlane against Vi is Kaisa/Vayne with a peel support.
Why Kai'sa/Vayne? Simple, Vayne with his E can interrump all her ganks and also kill her. And Kai'sa is a very annoying pick when she evolves his E, the invinsibity counters Vi.

Why peel supports? because one of the best things about Vi is that she can kill the adc by ulting. And if the adc has a Lulu/Janna/Tahm Kench/Leona/Nami, she will be easy to counter. Just use all the CC when she have alredy ulted.
If you pick tahm, you can do a trap. Wait Vi's ultimate and then use your W on your adc so she cant knock up quickly with the Q. A lot of Vi players use R and when they fall instant Q to do achain CC and a lot of damage.

I dont write top and mid counter picks because I honestly think she hasn't. Just with a bit of CC you can outplay.