Vector3.Angle

it is quite simpe if you know the conditions clearly.
This image illustrates it (i am using x/z axis):

the code would be like:

Code (csharp):

1.  
2. var A:Vector3 = Vector3( 0.0, 0.0, 1.0 ) // x,y,z
3. var B:Vector3 = Vector3( 1.0, 0.0, 1.0 ) // x,y,z
4. var angle:float = Vector3.Angle( A,B );
5.     // angle == 45
6.  
7. Debug.DrawLine ( Vector3.zero, A, color.yellow );
8. Debug.DrawLine ( Vector3.zero, B, color.yellow );
9.  

so what we do here is set the Vector A along the z-axis and the Vector B along the x/z plane.

The important thing to understand is that (if i'm not mixed up) Vector3.Angle always calculates the angle from the Vector3.zero to the actual Points.