Hi DrGonzo,
Could you please test the following query?
select
st.Name,
ss.Subnet,
ss.[Subnet Mask]
from vSiteSubnetMap vsm
full outer join vSite st on vsm._ResourceGuid = st.Guid
full outer join vSubnet ss on vsm.SubnetGuid = ss.Guid
order by st.Name
Regards,
Fábio Sanches