You might try something like -- this should handle vendors having no past cutoff,
or those having no future cutoff
"today" needs a suitable conversion to just date w/o time
comparison "=" may go on both, or one, or none Max/Min
"null" may be replaced by 1/1/1900 and 12/31/3999 in Max/Min
SELECT tblvendors.shipmentid,
tblvendors.vendornumber,
tblvendors.vendorname,
tblvendors.units,
tblvendors.shipdate
FROM tblvendors
LEFT JOIN
( SELECT vendornum,
Max( iif cutoffdate < today, cutoffdate, null) as PriorCutoff,
Min( iif cutoffdate >= today, cutoffdate, null) as NextCutoff
FROM tblcutoffdate
GROUP BY vendornum
) as VDates
ON vendornumber = vendornum
WHERE tblvendors.shipdate BETWEEN PriorCutoff and NextCutoff
ORDER BY vendornumber, shipdate, shipmentid