Is there any clever expression which can give me the Y rotation angle of the vector between velocity X and velocity Z. Or is the only way to write a custom expression for this?

Sort of like this:
http://www.euclideanspace.com/maths/algebra/realNormedAlgebra/complex/transforms/rotation2.gif [euclideanspace.com]

Found it! The atan2 expression does exactly what I wanted.