glaub ich machs so ..
view 1
SELECT nummer,num2, vkdat, max(UMZST) as umzst FROM table
group by nummer,num2, vkdat
order by nummer,num2, vkdat )

view 2
select nummer,num2,Max(vkdat) , max(umzst)from view1
group by nummer, num2

select UMZST from view2 where nummer='000000000310000006' and num2='9000000080' and VKDAT <= '20201202'

sollte funzen ...