SELECT
    COUNT(DISTINCT d.PONum) AS POCount,
    ISNULL(SUM(d.DocExtCost), 0) AS POValue,
    CASE v.Buyer_c
       WHEN 'Person5'
          THEN 'Person4'
          ELSE v.Buyer_c
    END AS BuyerID
FROM
    [Dbo].PODetail AS d
INNER JOIN
    [Dbo].Vendor AS v ON (d.Company = v.Company AND d.VendorNum = v.VendorNum)
WHERE
    v.Buyer_c IN ('Person1', 'Person2', 'Person3', 'Person4', 'Person5')
GROUP BY
    CASE v.Buyer_c
       WHEN 'Person5'
          THEN 'Person4'
          ELSE v.Buyer_c
    END
ORDER BY
    v.Buyer_c   <-- This order by doesn't work

GROUP BY
    CASE v.Buyer_c
       WHEN 'Person5'
          THEN 'Person4'
          ELSE v.Buyer_c
    END,
    v.Buyer_c

with cte as
(
    select count(distinct d.ponum) as pocount,
        isnull(sum(d.docextcost), 0) as povalue,
        case v.buyer_c
            when 'person5' then 'person4'
            else v.buyer_c
        end as buyerid
    from [dbo].podetail as d
    inner join [dbo].vendor as v on (d.company = v.company and d.vendornum = v.vendornum)
    where v.buyer_c in ('person1', 'person2', 'person3', 'person4', 'person5')
    group by case v.buyer_c
            when 'person5' then 'person4'
            else v.buyer_c
            end
)
select *
from cte
order by buyerid

SELECT COUNT(DISTINCT d.PONum) AS POCount,
   ISNULL(SUM(d.DocExtCost), 0) AS POValue,
   CASE v.Buyer_c
        WHEN 'Person5' THEN 'Person4'
        ELSE v.Buyer_c
   END AS BuyerID

FROM [Dbo].PODetail AS d

INNER JOIN [Dbo].Vendor AS v
    ON (d.Company = v.Company AND
        d.VendorNum = v.VendorNum)

WHERE v.Buyer_c IN ('Person1', 'Person2', 'Person3', 'Person4', 'Person5')

GROUP BY CASE v.Buyer_c
        WHEN 'Person5' THEN 'Person4'
        ELSE v.Buyer_c
     END

ORDER BY CASE v.Buyer_c
        WHEN 'Person5' THEN 'Person4'
        ELSE v.Buyer_c
     END