Try this (I don't have access to an NS to test, so I'm going from memory here):
SELECT vc.Name, vc.[IP Address], ISNULL(acid.[Client Date], 'Unknown') AS [Last Active Date]
FROM vComputer vc
LEFT JOIN Evt_AeX_Client_Logon logon
ON vc.Guid = logon._ResourceGuid
AND (DATEDIFF(dd, MAX(logon.[Login Time]), GETDATE()) > 30
OR logon.[Last Logon] IS NULL)
LEFT JOIN Inv_AeX_AC_Identification acid
ON vc.Guid = acid._ResourceGuid
WHERE DATEDIFF(dd, acid.[Client Date], GETDATE()) > 30
AND vc.IsManaged = 1