Moved "sDate is null" to outside the bracket and that worked.
Select Table1.vId,Table1.sId,sUnits
from Table1 Left Join Table2
ON Table1.sId=Table2.sId
where (
sDate=(SELECT MAX(sdate) FROM Table2 AS S
WHERE S.sId = Table2.sId)
OR sDate is null)
and vId=1