I believe the problem occurs because you cannot make a method type declaration nullable. Can't say that I've ever tried, so if I am wrong here, I'd appreciate someone else letting me know. You will have to have a field that is nullable, and a Vector3 method that returns a Vector3:
Is there a specific reason that you need it to be null and not just (0, 0, 0)? Changing the last line of the return statement from 'null' to Vector3.Zero will get rid of your error, but the vector3 will definitely not be null.