Keep the Dragon warded at all times. Vi has good objective control and take them pretty easily. One way of countering this is by keeping the objectives warded or taking them before she gets the chance to.
As Vi is an early game Jungler, try to place vision around her jungle entrances and in the river to reduce the chance of her getting successful ganks off. Vi can gank from level 3 onwards once she has her Q.
Try your hardest to equal Vi’s pressure by ganking often or counter ganking her. If you’re not as good as her in the early game, try to secure objectives if she shows on the opposite side of the map or invade her jungle and steal away camps.
Keep in mind that Vi has great pick potential with her Ultimate R. Avoid walking too far forward in a team fight as she may pick someone off who has stepped out of line.
Depending on Vi’s build path, she could look to assassinate targets that are walking around Summoners Rift alone. If she is ahead with lots of damage, do not go through the river if it’s not warded.
If Vi initiates a fight with her Q, try to burst her down before she is able to activate her UltimateR. Even though she is tanky, she is prone to burst damage and CC.
At level 6, Vi will get access to her Ultimate R. Whenever this ability is up, her ability to gank increases dramatically.
When Vi hit’s level 3/4, she will start to gank thanks to her having access to all her basic abilities. Expect her to start ganking around this time.
As soon as Vi gets Trinity Force, her damage output and kill pressure increases a lot. Avoid 1v1’s with her unless you have a clear advantage.