The following query will display those computer assets that either do not have a department or an owner associated with them:
SELECT vc.Name AS Computer,vado.Department,vauo.[User Name] AS [Owner] FROM vComputer vc LEFT JOIN vAssetUserOwner vauo ON vc.[Guid] = vauo._AssetGuid LEFT JOIN vAssetDepartmentOwner vado ON vc.[Guid] = vado._AssetGuid WHERE vado.Department IS NULL OR vauo.[User Name] IS NULL ORDER BY vc.Name ASC