Hi my name is Como,

I am a hardcore LoL player and have been gaining alot of experience in the game.
Vi is one of the strongest junglers at the moment, even after the small nerf in 4.4.

Pros

Has one of the best initiations you can wish for
Can easily shred through armour due to her W + armor pen runes/items
Great at ganking, counterjungling and dueling

Cons

Long cooldowns early game
Skillshot dependend
Enemys can flash over walls when you ult them, For example: you ult the enemy ezreal, he flashes over the wall into a turret. You will always follow, so know when to use your ult.

How to play in ranked:

In ranked you want to make sure to get a pull from botlane, then go to your wolves, or wraights.
(always go to the camp further away) You want to go to the further camp to let your E wich you should level second can get its 2 stacks.

How to gank:

After you take your second buff you want to make sure to gank toplane, obviously the enemy jungler could do the same, so if your top didn't, make sure to ward tri-bush or the bush itself depending on the sides. When you gank you want to make sure that you hit your Q, if you miss this theres only a small chance of getting a kill. So if you miss leave the lane. If you do get a kill: ALWAYS PUSH THE LANE Low elo players might get mad at you for "stealing cs" but you want your top to get ahead in xp/farm so you want to deny your opposing top laner farm.

How gank with your ult:

Vi's Ulti is a long ranged dunk. You want to make sure not to just hit it when you can.
Once you become an experienced player you will be able to predict flashes. Saving your ult and perfectly timeing it can secure you a kill.

When can you start dueling the enemy jungler?

Depending on the enemy jungler you should always be able to duel. Just pay attention if lanes are missing or not. I usually try and ward the wolves, wich is easy accasible due to the fact that there are so many bushes that provide perfect cover.

You want to smite the wolves as soon as they are low enough, if your experienced with vi you can do the following combo with her: Q-smite-basic attk-E-basic attk-E.

Teamfights

In general, Vi will play the role of the initiator / front line. Her skill set makes her incredibly durable as well as very good at killing tanks (due to the ridiculous armor shred from W) so having her peel for your AD carry is a decent playstyle for Vi. Additionally, due to the diving nature of her ultimate, you can also utilize her to disable any enemy champion and easily take them out of the fight. Try to reserve your ultimate for their squishy AD/AP carry but use it at your discretion when the situation calls for a good knock up or when they are out of position. If you have built full tank and have high CDR, you can exclusively peel for your AD (usually after you have used your Ult to engage on to the enemy carry) as your Q will have a very low cooldown and can deposition enemy bruisers, and your ultimate can also easily shutdown an assassin recklessly going into your team.