Many people, especially in lower rankings, think that they're the best and need to carry their teams if they want to climb out of (Insert rank here). Mathematically speaking, you have an 80% chance of being wrong. But that doesn't have to stop you from being a massive contribution to your team. League of Legends is a team game, and the best way to win is to work as a team. This means grouping up and jumping on the poor chumps that are on the wrong end of the map and obliterating them, ending their feeble lives for 20-70 seconds. This means you need "Engage", or Initiation. One of the strongest initiators in the game is Vi. With her ultimate, she guarantees one person on your team is going to have a very bad day. She's tanky enough that she'll survive being in the center of their team, but she does enough damage that she demands being peeled and focused or there will be a dead carry soon.
Pros / Cons
Strong Feral Flare jungler
Strong ganks after 6
Tanky enough to take a good hit
Does damage, even if built full tank
Ultimate assures an engage on the carry
Sometimes too manly for her own good
No natural sustain
Can fall behind, in which case she just kind of jumps on someone and dies
Her ultimate puts you behind enemy lines without Tom Cruise
Greater Mark of Attack Damage
Greater Mark of Critical Chance
Greater Seal of Armor
Greater Glyph of Magic Resist
Greater Glyph of Cooldown Reduction
Greater Quintessence of Attack Damage
Until I test HP yellows, armor yellows won't steer you wrong. Jungle creeps do AD damage meaning the more armor the better.
Since Jungle creeps do AD and not AP damage, MR is only useful when ganking an AP champion. As such, I prefer to only take 3 MR runes over 9 and get some CDR runes, so you can gank more and clear faster with [Excessive force].
The 21 points in defense is standard. Not much to say there.
The 9 points in offense is also fairly standard, only thing up to debate are the first 5 points. I love CDR, something I picked up from my supporting days. AS is huge on Jarvan, so I put the 3 points in there, but at the same time CDR is necessary so I put 2 points in the CDR.
Boots of Mobility
The Black Cleaver
Locket of the Iron Solari
[Spirit of the Elder Lizard] is my preference. I've tested [Feral Flare] on a lot of my junglers and at first I was like "Oh Flare OP" and it is a very strong item and it can work on Vi, but now that they've nerfed it and I've tested it, I have to recommend Spirit of the Red Buff. I love the jungle sustain so so much, and Vi needs to be ganking the lanes, not farming for days.
[Boots of mobility] Are ganky boots. Ganky boots are best boots, or at least until about minute 20.
[Black cleaver] is the item Vi was built around. CDR, HP, armor shred and flat armor pen. How could you not?
[Frozen Mallet] is my favorite item in the game, and is completely optional. It's the manliest item in the game and I love it, but if you don't share my love that's quite alright, go swap it out for something out.
[Randuin's Omen] is the standard armor item of any tank. Lots of armor, 500 hp, an amazing passive that makes ADC's and other [Feral flare] junglers weep. And the icing on the cake, the AoE slow that scales off of armor and MR. It's pretty hot.
[Locket of the Iron Solari] in my opinion got a huge buff now that it builds out of [Negatron cloak]. It's a strong MR item that gives a bunch of other cute stats, and a team shield which I love. Can be swapped out with [Banshee's Veil] if they have an ability you desperately need to not get hit by.
[Blast shield] Whenever an ability hits an enemy, Vi gets a shield equal to 10% of her max HP, on a cooldown of 18/13/8. This is the closest thing Vi has to jungle sustain, and also helps take the initial burst when she initiates in a gank or a team fight. This shield gets big late game and if you're clever you might even pull off a few Dunkey moments.
[Vault breaker] This is Vi's escape and initiation. It charges up and after a little over a second it reaches full range, which is about the range of an ADC auto attack. It knocks back a tad on impact, which gives enough time to get an autoattack off. If you have Red/FroMall (hopefully both), you'll have enough slows to make even WildTurtle have trouble kiting you. Max this first to increase burst and reduce cooldown.
[Denting Blows] Like [Teemo]'s poison, you put points in it and forget about it, as your enemies die beneath your fat hands. Every third hit, Vi gets an AS steroid, shreds some of their armor, and deals a chunk of % max hp. It doesn't scale too well with level though, so it's good to max this last.
[Excessive force] is an Autoattack reset that deals damage in a cone, this is how you clear fast and murder the small creeps in the camp. Your gank/murder combo is Q in to a sucker, auto attack, E, autoattack, E again, auto, ded. This full rotation get's two procs of your [Denting blows] off, which at rank 1 with no ad at all is 8% of their max health, shreds their armor for 20%, and gives you 30% AS. Your E holds up to 2 charges and increases in damage and reduces cooldown as it level so this is maxed second.
[Assault and Battery] This is what makes Vi borderline OP. Her ultimate points out someone on the enemy team, rushes to them and dunks them. Any chump in the way gets knocked aside and takes 75% that damage. This can't be blocked, peeled, dodged, nothing. Basically, you press R and you're punching the chump you targeted. Your initiation is ideally just this, you want your Q to murder your target, but you can Q in to range and then ult.
Creeping / Jungling (Early game)
Take W first, since it will do the most damage to the buff, and after you kill it grab E. I prefer starting blue buff, but starting Red is acceptable since she is an AA based champ. I like blue buff because spamming moves in the jungle gets you through fastest, and you can use all the CDR you can get. Grab wolves, then head to red buff and butcher it while its children watch, putting a point in to Q.
Vi has gank potential pre-6 thanks to her Q knockback/dash and red buff, but they're not too difficult to escape from. If you see a good gank, go for it, but getting 6 is your priority. Get [Wriggle's lantern] whenever you can, but as soon as you hit 6, find a gank. If the enemy is pushed, they're dead. If the lane is centered, probably still dead. If the lane is frozen near their turret, chance of them being dead. If they're UNDER their turret, they actually have a very good chance of being made dead because they can't juke your ult and dance under their turret. So basically, kill em.
Team Work (Mid Late game)
Like most jungles (or at least the ones I write guides for), your job is to start the fight. There's not much to it, when your team is ready (and hopefully theirs is not) you ult their carry and punch them to ded.
Vi is one of the manliest champions in the league of legends, and is borderline OP due to her extreme power, especially after 6. She shreds carries and tanks alike, and can be a huge contribution to a winning game.