You should start by maxing Q after leveling all your abilities one time. This makes sure you can fly to them quickly and make some fast damage after landing 3 hits. You can start your harass on level 3 with this build.

I've played Vi mid a couple of times now, all scores ending in 7> KDA. The trick is to harass early by using your 3 skills into making 3 hits and then run back behind the creeps waiting for your cooldowns to refresh.

The reason Vi is so great is that she has a high damage physical burst early game. She can jump with her Q which can easily be combined with Flash. You have a great ultimate that negates crowd control and overal Vi scales really well into lategame, unlike other offtank characters.

First version of this guide so feedback and testruns would be great! Enjoy guys.

1.1: Added optional items & added a note to build sequence.