You can write it in simple way using OR keyword..
The below query will get you results where @vcid is null or is blank or it matches with column value. It might not what you asked but try this, it will work
Select *
FROM FWB_SF_Opportunity B
left join FWB_VWSFData A ON A.opportunityid=B.id and ReviewMonth=@ReviewMonth
AND A.VCID = isnull(@VCID,A.VCID)
WHERE ( @vcid is null or @vcid ='' or A.VCID = @VCID )
If you still want in case statement, then try this
Select *
FROM FWB_SF_Opportunity B
left join FWB_VWSFData A ON A.opportunityid=B.id and ReviewMonth=@ReviewMonth
AND A.VCID = isnull(@VCID,A.VCID)
WHERE A.VCID=case when @vcid <> '' then @VCID else null end