Thing with vi is that full off, full tank and bruiser are all viable. It mostly depends on teamcomps and also on the sitation.
Tanky builts tend to be safer and more gold efficient, so they work better then you are behind.
Full off builts are usually more risky, but are great to push an advanrage.
For masteries, Thunderlords if you plan on building rather agressive, Grasp if you go more tanky and thus get the hp for the scaleing.
For runes, it depends on what items you built. Your aim should always be to reach 40%cdr(or 45% with the masterie) so running cdr/lvl glyphs for 10% cdr a 18 should be considered. With cdr blues, i usually go for hp/lvl yellows as it helps vs physical and magical damage
I also prefer AS reds if faceing more tanky opponents or builting rather tanky my self as she relies alot on her W dps for drawnout fights. Else, i prefer arpen as i synergieses well with the armor shred on her W.
For items, nearly anything can work on vi, though i recomand getting atleast one of the hydras and a sheen item. Black cleaver vs tanky opponents also is a great item for her.
If going tannk, you don't need a sheen item as you won't burst anyone anyway. Frozenmallet however makes you very annoying to deal with.
Also, you need to adjust her playstyle to her built. If you built full off, you have to play her like an assasin and either splitpush or stay a bit awawy from the fight and wait for going in till some major cds are blown, else you die very quickly in a teamfight.
As bruiser, you also don't want to be the first to engage unless you get a great flank. U usually want to go in as follow up on your tanks engage. Bruiser Vi also os a powerfull splitpusher.
With tank Vi, you are the vanguard of your team, meaning you dive head first into your opponents to lock down thei carry. You most likely lack the damage to kill him by your self as tank, but with proper followup that doesnt matter.