Not Updated For Current Season
Remember that whatever you do, Vi is a bruiser. If you build no damage, she will be useless and if you build no tank she will be squashed before doing any damage. Vi relies very heavily on her Q for ganks, escapes and general brawling.
How to execute her combos
Vi's combos are more complicated than it seems on the surface. Her easy to learn, easy to execute combo is just Q-auto attack-E and you proc her W. Very simple, very effective. To start teamfights as Vi I always prefer to save my ult for locking down escaping enemies instead of engaging. Initiating a teamfight for me looks like (charge Q)-Flash-(release Q)-aa-E-aa-ult-aa-Q and that will kill any squishy target while not even allowing them to use any abilities.