Look back at America West ---> US Airways
I had Idine accounts with both America West and US Airways before the merger. After the merger my idine accounts are still seperate, but both feed into the same merged Frequent Flyer account.
The new US Airways (several months after the merger) took care of merging the actual FF account. Idine kept their accounts serpeate but both feeding into the new (same) FF account.
I would have to assume the Northwest / Delta merger would be similiar, so there is no need to worry about VIP status.