The Source Volume Group on the Choose Backups tab spans all steps of a Vault profile (most notably Duplication and Eject). However, if you do not duplicate images, you do not have to specify a source volume group (the Source Volume Group field is ignored). Conversely, the Source of Backups... field on the Duplication tab applies only to the Duplication step.

Even if no images are selected for duplication, images can still be ejected if they are in the Source Volume Group and in an off-site volume pool specified on the profile Eject tab.