LoL this is on liquipedia:
“Vipers is Weak against:Viking, Mutalisk and Phoenix”
in game viper is i bigest counter to thees units 
Viper is like old liberator … counter siege units … And had OP aoe vs air . they realise that was mistake … and nerf that air Aoe to the gound … but why they didn’t nerf viper same way ? Viking, Mutalisk and Phoenix should counter viper …