Could you use this vanilla bean buttercream as a macaron filling or would it be too thin? m I

I don't see any reason why you couldn't use it as a macaron filling. Those are usually filled with some sort of buttercream. You can control the thinness or thickness simply by adding more or less milk as you beat it together. The liquid is just to give it a spreadable consistency, and it's up to you how thick or thin you want it.