
Vanilla ice cream is a churned, frozen dairy dessert made from sweetened cream and milk, flavored with vanilla.
A substitute has to cover three things at once — sweetness, dairy and the ice itself. Miss one and the drink has to be rebuilt around it.
Gelato
Less air, more density, served warmer, and in a blended drink it behaves like a richer, heavier ice cream and needs a splash more liquid.
Soft serve and frozen dessert
Often not ice cream at all by legal definition, with vegetable fat in place of cream, and it melts differently and tastes flatter in a drink.
Frozen, closed, and never refrozen after melting: the ice crystals grow coarse and the texture goes grainy for good. It also absorbs freezer smells quickly, since ice cream is mostly water and air with fat around it.