vec_diff ( vtemp, you.x, my.x ); 
vec_rotate ( vtemp, vector ( 0, AngSpeed * time_step, 0 ) );
// or
// vec_rotateaxis ( vtemp, vector ( 0, 1, 0 ), AngSpeed * time_step );
vec_add ( vtemp, my.x );
vec_set ( you.x, vtemp );