It's entirely possible you never find the same fare on nwa.com.
It's also entirely possible nwa.com winds up cheaper than delta.com in the future as DL starts selling seats in a higher fare bucket but nwa sells them in a lower bucket.
BTW, I believe calling NW will help you here. They should be allowed to sell you an entirely DL-operated, DL-coded ticket and allow you to apply your NW vouchers.
NW can't do that with CO but they should be able to with DL.
-RM