title: Signed angle between two 3D vectors with same origin within the same plane
body: <p>What I need is a signed angle of rotation between two vectors Va and Vb lying within the same 3D plane and having the same origin knowing that:</p>
  
  <ol>
  <li>The plane contatining both vectors is an arbitrary and is not parallel to XY or any other of cardinal planes</li>
  <li>Vn - is a plane normal</li>
  <li>Both vectors along with the normal have the same origin O = { 0, 0, 0 }</li>
  <li>Va - is a reference for measuring the left handed rotation at Vn</li>
  </ol>
  
  <p>The angle should be measured in such a way so if the plane would be XY plane the Va would stand for X axis unit vector of it.</p>
  
  <p>I guess I should perform a kind of coordinate space transformation by using the Va as the X-axis and the cross product of Vb and Vn as the Y-axis and then just using some 2d method like with atan2() or something. Any ideas? Formulas?</p>