You can get an angle between two vectors using this method:

1. Calculate cross product of the vectors
2. Get look rotation of both vectors with Up equal to the cross product
3. "Subtruct" these rotations
4. Get Y component of the resulting rotation (X and Z will equal to 0)

Note: when you calculate the cross product and "subtract" the rotations, the arguments should be passed in the same order. Otherwise, the angle will be a negative number.

Script example: