This is my very first guide. Honestly, I am just making it so I can keep track of my own ideas for a build. About me: I've been player LoL since season 2 and I have my moments, but overall I am not very good.
I recently stumbled upon Vi and found that she is really fun to play with and is all around a good champion with straight forward abilities. My main meta is jungle, so I have only ever tried her in the jungle, but to me I think that she makes the most sense in the jungle. She has 2 gap closing abilities that are good for ganking during the laning phase. Also, she is a very strong duelist which means she is difficult to counter jungle. Vi is strongest in team fights. The main goal in team fights is to disable the either the AP or AD Carry with the ultimate and then to tank damage. Afterwards Vi can either escape to survive or chase down feeling enemies. Vi shines in the mid and late game.
BTW I am still in the process of building the guide so there will be mistakes. You probably aren't going to actually read my guide anyway.
Passive Blast Shield: This passive is straight forward since it is automatic. It is best to make sure that the passive is not on cool down when initiating a fight. A 10% max health shield can be very useful. Note that cool down decreases with level so at high levels this will be proc much more frequently. If laning then this ability could potentially be used to bait enemies when at low health, however, this is likely a very rare opportunity.
Q Vault Breaker: I usually take this ability at level 2, but W is also a viable option. Q is a great tool used for chasing and escaping. Practice using it to jump through walls. Max this ability second by level 13 for the reduction in cool down.
W Denting Blows: This ability is wonderful because you do not even have to think about it, it is all automatic. Max this ability last because only the damage and attack speed bonus are increased. Also, this ability scale better with more AD, so it is not vital to put points into it until you start building heavy AD items, which is generally later in the game.
E Excessive Force: Take a point at level 1 and then max E first by level 9. This ability is wonderful for clearing jungle and also increases Vi's damage. Max first to increase damage and decrease cool down for maximum effectiveness.
R Assault and Battery: Super straight forward ultimate that is true ultimate. It's main purpose is used for initiating. If this ability is up and there is a team fight or you are ganking make sure you initiate with this ability. First, it serves as a gap closer to put you in range or your target. Additionally, the 1.25 second is the perfect amount of time for your teammates to catch up with you and nuke your target.
Combo: REEW is the main combo (notice W proc automatically on your third attack. Keep the same target until they die to continue procing your W. Since you initiate with R, you should only use Q to chase down a feeling target, escape if you are low health, or move to your next target if you have successfully killed your first. Note that it is best to time your ultimate to avoid CC effects if possible.
1) Use a standard Jungle start with Hunter's Machete and 5 health potions.
2) On first back, pick up boots and Madred's Razors. Vi is not the strongest ganker before level 6. The most viable times to gank are when the enemy has push a teammate's lane. Therefore, it is best to get boots and Madred's to be able to clear the jungle quickly and at least be open to the possibility of ganking if an opportunity presents itself. Note that you will not be upgrading to Wriggle's Lantern. Wriggle's Lantern does not scale well at all late game and is really a waste of a spot. Therefore, sell Madred's Razor sometime mid/late game when you need the space or the gold to continue your build. By this time you should be able to clear the jungle/dragon so fast even without Madred's Razor.
3) Boots of swiftness are best for ganking and getting around the map. With Vi's gap closers, boots of swiftness make chasing very easy. You make also opt for Merc Treads as they are also a good choice if you need the MR or CC reduction, however, note that you will be buying Zephr later which will compensate not having CC reduction for Merc Treads.
4) Next, rush Black Cleaver. All of stats for the Black Cleaver are beneficial for Vi. The AD scales with the abilities, the CDR is usefully for skills, and the health benefit scales with passive.
5) After Black Cleaver is complete buy a Giant's Belt. You may be asking why I don't build more AD or AS. The reason is simply that Vi is best used to assist your team to get kills. Yes, I know you want all the kills and Vi can do it, but it is best to allow your teams to get the gold instead. Of course you are asking why, the main reason is that Vi is a very efficient farmer and it is likely that you will rival the CS of your ADC. Since gold is not too much of an issue, it is best to build health to increase sustain. Eventually, build you belt into Warmogs to increase health and sustain between ganks.
6) Now that you have complete the core build Vi is a very efficient champion. Between ganking make sure that you are clearing the jungle to keep your CS up. If teammate are dead or out of their lane then use QEE combo to quickly clear enemy minion waves. Buy a Ravenous Hydra is very useful for its AOE. Additionally, the life steal increasing sustain. Don't forget to spam its active. Since you never use your W you may want to place it in the 2nd item slot to increase spammability. I personally use they mouse buttons to item activation in slots 1 and 2.
7) Finish your build with Zephyr and Randuin's Omen. Zephyr is the perfect time for Vi, its got AD, AS, CDR, move speed, and CC reduction. All stats are essential. Since you are often initiating battle with the enemy team Randuin's is a good item for tanking damage and slowing the enemy team. Note that this item is way to expensive if you are not using the active part of it. So if you don't plan on using the active then purchase a different item instead.