Pick last Vendor Number (From Master table ) who sold Item number(From detail)
You can use ROW_NUMBER() function to rank each item by it's date, being 1 the most recent one.

;WITH TimingRankingByItem AS
(
    SELECT
        P.*,
        TimeRanking = ROW_NUMBER() OVER (PARTITION BY P.ITEMNO ORDER BY P.DTARRIVAL DESC)
    FROM
        PORCPL AS P
)
SELECT
    T.*
FROM
    TimingRankingByItem AS T
WHERE
    T.TimeRanking = 1


Or using GROUP BY you should join by both the item number and the date, but if you have 2 max dates with the same item you will see both.

;WITH MaxDateByItem AS
(
    SELECT
        P.ITEMNO
        MaxDate = MAX(P.DTARRIVAL)
    FROM
        PORCPL AS P
    GROUP BY
        P.ITEMNO
)
SELECT
    P.*
FROM
    MaxDateByItem AS M
    INNER JOIN PORCPL AS P ON
        M.MaxDate = P.DTARRIVAL AND
        M.ITEMNO = P.ITEMNO