The user's manager is a resource association so you need to join to the ResourceAssociation table. This is a sample query to pull user name with manager:
select
vu.[Given Name] + ' ' + vu.Surname as 'User Name'
, ISNULL(man.Name, 'No Manager') as 'manager'
from
vUser vu
LEFT JOIN
(SELECT ra.ParentResourceGuid, it.Name, ra.ChildResourceGuid
FROM ResourceAssociation ra LEFT JOIN vItem it
ON ra.ChildResourceGuid = it.Guid
WHERE ra.ResourceAssociationTypeGuid =
'049C633F-8413-42AE-93EA-F4EB7EDAFC65') man
on man.ParentResourceGuid = vu.Guid
I LEFT JOIN with parantheses to make sure that my results include user names and managers (if any). If you don't left join then the query will only returns users who have managers, fYI.