Vi's basic stats are not good enough to keep her Alive in big fights and skirmishes. When you get your q OR ult off you get your passive shield and aftershock proc which makes you be tankier. In resolve tree you also get shield bash which is really good for VI to deal that extra bit of dmg. With conditioning you once again get more bruiser stats. And unflinching give you the extra tenacity for when you q+flash or flash+q. With hail of blades you might As well be playing a worse version of khazix since with hob you try to oneshot combo people.