Okay then! Let's say we are ONLY working in the 1st Quadrant of the x-y plane. I have some vector in that region that makes some angle with the +x axis. I then rotate that vector through some angle. The initial points of the vector before and after are coincident, i.e. I only move the terminal end to some new point while maintaining the vectors magnitude. I know the initial point, I know the terminal points of both the original and new vector. I now need the angle that I must have rotated through to get to the new point. Is this a dot product problem?