Pure builds are the only viable thing in this game, splitting is useless because you don't really deal damage nor are you tanky enough to take a beating. With the current dhans penetrating any form of damage drop and rekking you with invenom, guards combos and I dont even wanna talk about the ridiculous amount of debuffs you have to accumulate single target dmg like rangers anti pot, ulti skill, broken rumir loop and noir accumulate dmg. There is nothing else you can do as support but to go full vit and hope you survive more than a few hits and you get your debuffs etc. off on whoever is your opponents main dps...
For damage dealers its the same thing, you either go full agi or full str etc, adding some vit is useless, you'll get shotta'ed anyway in a few hits if your supports dont protect you with perm lvl 7 obli, loop and what not so you better maximize your damage output.