In the early game, Vi struggles heavily with the inability to set up her damage if she misses Vault Breaker . Having any amount of mobility will make Vi borderline useless because she cannot get off Denting Blows in a timely manner. Hold on to your dash until she is closer to escape.
Due to Vi's heavily long cool-downs, she usually does fairly well in short bursts where she lands her Vault Breaker or in very long trades where she can apply one or two shields from her Passive. Kit around her and keep your distance to limit her damage output.
Vi will oftentimes attempt to Flash-Vault Breaker onto an enemy to catch them off guard. When you see Vi come out of the fog of war with a fully charged Vault Breaker , prepare to Flash.
Coming into mid game, Vi finally becomes a force to be reckoned with. Having lots of vision to ensure that you do not get caught by her Vault Breaker into Assault and Battery combo is essential to maintaining tempo and avoiding unnecessary deaths.
Vi's win condition during this time period is usually to Assault and Battery the enemy ADC or Mage. Positioning is key here in order to defend your carries from this extremely telegraphed .
Vi typically requires a large amount of gold to properly do her job. Most solo-queue Vi players will go Trinity Force which costs a huge amount of gold and leaves her relatively squishy for a bruiser. Make you have the ability to burst her down before she is able to one shot your ADC.
Level 6 is the first power spike. This will enable her to deal an extreme amount of damage and increase her kill threat. Usually, Vi will immediately path towards bot side to get a pick on the enemy bot laners and grab a free Dragon. The easiest counter toward this is ward and don't die.
Vi's second major power spike is Trinity Force. She becomes a borderline assassin, able to deal huge amounts of damage toward any squishy target. But, she is able to be killed very quickly by high burst damage and she will be really squishy.
Vi's final major power spike comes from the item combination of Sterak's Gage and Guardian Angel. This dynamic item duo allows her to dive without fear, tank a massive amount of damage, deal huge amounts of damage. That being said, she still is extremely susceptible to crowd control.