It is only me who has a strong feeling that this vector is an ugly construct here - [Node #tree/Tree Empty, #tree/Tree [Leaf 42]] and traditional S-Expression, which just describes an abstract structure - a list, not a particular data-structure - vector, is much better way?

And [Leaf 42]? What is this? Vector? Of elements of arbitrary type?

But a list of arbitrary elements is a quite natural and human-mind-friendly concept, and the Lisp notation, which intentionally omits any description of underlying representation, is a great idea.