Hi,
if I remember correctly the role-based administration will not display entries in the left nevigation tree which are not assigned to a specific user/group. For example if you have no rights to administer your vaults than you will not be able to see them.
As I know it is not practically possible to create a read-only role for RBA. Why should you allow access to the Vault Administration Console if the person accessing it can't do anything?
With kind regards,
Nico