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.

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.