SubArea.VectorIconSpecifies a URL for an vector image to display for the SubArea.
Group icon is currently unused according to the documentation, but it might be worth adding as it is in the schema.
Group.VectorIconUnused. It is not possible to display an icon for a group.
You recently added VectorIcon attribute to Areas.
‘VectorIcon’ attribute can be applied to SubAreas as well as Areas to display an Icon in the Unified Interface. See https://dynamics365northernireland.com/2018/09/10/adventures-in-uci-site-map-configuration-and-usability/