Vi fits the role of bruiser very well and should be played as a semi-tank. Abusing your Vault BreakerVault Breaker would be a good harassing strategy as it closes the gap between you and your enemies. Be careful about using this ability as Vi has no other escape mechanisms once Vault BreakerVault Breaker is on cooldown.
Because Assault and Battery does 75% of the ability's damage to enemies between Vi and her target, try to hit as many champions on the way to the target as possible.
Vault BreakerVault Breaker can be used to dash over almost any wall in the jungle. You can dash over walls to gank unsuspecting players, or, with some right warding, easily catch the enemy jungler off guard. Using Vault BreakerVault Breaker will also allow Vi to reach her jungle camps faster.
The Black Cleaver has great synergy with Denting BlowsDenting Blows's armor reduction, reducing the enemies armor by 40% total. This will greatly improve both your damage, and physical damage dealt by your teammates to the target.
As Vi has no natural sustain, investing in lifesteal might be a good idea. Both The Bloodthirster and Ravenous Hydra have high attack damage and decent lifesteal, and are great offensive items for Vi.
Zeke's Herald can also be purchased. Although is doesn't have as much attack damage, the extra health and cooldown reduction make it worth your gold.
Despite you should build Vi as a attack damage caster, getting some attack speed will greatly improve her effectiveness. Due to the higher attack speed, Denting BlowsDenting Blows will proc faster and Vi's overall damage will increase.
Wit's End is a great item for attack speed, as it also provides magic resist and a decent on-hit effect.
When not building Mercury's Treads, the Zephyr should be considered. The Zephyr offers Tenacity, which is highly favored in the current meta, some attack damage and cooldown reduction and a decent amount of attack speed.
Getting some cooldown reduction helps Vi charge her Excessive ForceExcessive Force faster. While one should not focus on getting maximum cooldown reduction, getting one or two items won't hurt.
The Brutalizer is a great early game item. It has decent attack damage, some nice armor penetration and some cooldown reduction.
The Guardian Angel is a good item for Vi's aggressive playstyle, as it will allow you to die once in a teamfight. As Vi is a great initiator, she's likely to be focussed down quite fast. Getting a Guardian Angel will allow you get back in the fight.
Vi relies heavily on crowd control to win 1v1 fights or 3v3 fights. She can easily disrupt enemy lines with her ult, and this will give her teammates enough time to back her up. Scattering is a good way to prevent Vi from knocking you aside or dealing massive damage with Excessive ForceExcessive Force.
As Vi is a powerful bruiser, making sure to pin her down and not letting her get the core items she needs would give you an advantage against her team as she would be too soft to initiate and her damage output would be very low.
Keeping Vi at bay can win skirmishes or clashes, as she has long cooldowns and very few escape mechanisms.
Not letting Vi proc Denting Blows is crucial for lane control and poke, so watching out and avoiding a Vault Break can often decide the fate of a skirmish (being CC, a Denting Blows stack and gap closer).
Early game Vi can easily be pushed out of lane by controlling her shield procs and attacking after it expires if she uses Excessive Force to farm.