Unfortunately for Vectorworks 2018 is planned to be able to attach only the video screen. If you need to include the projector properly in the calculations, you need to set it's location via the OIP parameters for Projector distance and vertical position. Note that Braceworks calculations don't need a Hanging Position, it's meant to allow hang with offset (via the Witness Lines) and also object associations. In the attached image no Hanging Position is used.