Does the VigorACS 3 Cloud subscription allow hierarchical group management within a single tenant, including the creation of sub-groups and the assignment of a group administrator to each group?

No, this feature is not available. If you need it, please contact support@draytek.com.au and we will do our best to assist you.
If you need hierarchical group management within a single tenant, such as creating sub-groups and assigning users to groups, we suggest using the standalone, self-hosted VigorACS 3. This setup includes the Group Management and System Management modules.