Попробуйте такой запрос (я тут правда успел убрать те 2 таблицы, которые посчитал ненужными):
X++:
select ITEMID from VENDSPLISTTRANS
group by ITEMID
join VENDSPLISTJOUR
where VENDSPLISTTRANS.INTERNALSP == VENDSPLISTJOUR.INTERNALSP
and VENDSPLISTJOUR.SPDATE >= 25\09\2013
and VENDSPLISTJOUR.SPDATE < 02\10\2013
outer join PURCHID, INVOICEDATE from VENDINVOICETRANS
where VENDSPLISTTRANS.INTERNALSP == VENDINVOICETRANS.INTERNALSP
and VENDSPLISTTRANS.ITEMID == VENDINVOICETRANS.ITEMID
group by INVOICEDATE, PURCHID