Vi is a very easy champion to play. Her game all around is very strong. Early game she will win against 1v1 buff steal attempts. Mid game she deals TONS of damage and late game she helps the team get the kills in team fights. This is a very good champion to pick up.

You start at Red Buff if the other jungler starts at blue.
You start at Blue Buff if the other jungler starts at red.

The reason for this is so you can avoid the 1 v 1 if the other jungler does try to counter jungle you. Even though you win 1 v 1 early game it is better to be safe than feed first blood.

When you play Vi you never want to initiate with your ultimate. Your ultimate should be the gap closer if you are going to finish off the enemy you were fighting. You initiate with your Q then auto attack once and E and E again. After that your enemy should be fairly low and try to flash out. Since you didn't use your ultimate you have it as a finisher and it should be a guaranteed kill.

In team fights, always make sure you use your ultimate skill on the carries. You don't want to use it on the tanks because you really will not be helping anybody but the other team if you do.

For ganking, do not initiate with your Q or Ultimate. if you can try to run up to them and use your E. Q after when they are running away. just so you can deal the maximum amount of damage. If you get the kills during the gank it is okay because Vi is a very hard snowballing champion.

Pros
Deals a lot of damage
Guaranteed kill during ganks if played right
Easy to Learn

Cons
Can be very squishy up until you build the Mallet
Can get picked off really easily if your team doesn't follow up