Hi
Vector3d.GetAngleTo method is overloaded
vec1.GetAngleTo(vec2) always returns an angle in the range [0, pi]
vec1.GetAngleTo(vec2, referenceVector) returns an angle in the range [0, 2*pi], where referenceVector is the normal vector of the reference plane.
Following your example, if the referenceVector is (0.0, 0.0, 1.0), according to the "right hand rule",
(acVector2.GetAngleTo(acVector1, referenceVector)) * 180 / Math.PI
returns 135.0
(acVector1.GetAngleTo(acVector2, referenceVector)) * 180 / Math.PI
returns 225.0