How to check if ALL vassals have positive opinion of liege

[Captain Gars]

I believe this should work:

		liege = {
			NOT = {
				any_vassal = { 
					higher_tier_than = baron
					NOT = { opinion = { who = PREV value = 0  } }
				}
			}
		}

Using LIEGE as you did in your example is a special case that only works in very few places. Use the previous scope (prev) instead. And tier = X, unlike other triggers, does not mean >= but just equal to. Use the higher_tier_than instead.