I am not so sure if the VS ticket will be rebookable as the DL system itself may reject any attempted booking of an AA segment.
For example, LH and BA interline, a passenger may fly SK on an LH Miles and More ticket, but then misconnect in LHR. SK still won't be able to rebook on to BA as their reservation system won't issue a ticket for any airline they don't interline with, even in an irregular situation and even if it's not their ticket originally.
It all depends how it's set up, but in my ticketing experience (4 airlines across the 3 alliances, on 3 different reservation systems), if there is no interline agreement then the system physically won't even let you book and issue new tickets regardless of the reason, even if you try and trick it into taking a codeshare flight number. A FIM would be rejected by the accepting carrier if no interline agreement exists too, so even manual ticketing is a no no.
AA/DL must have a work around though as they are honouring tickets issued up to the date of the termination. Quite how that would go down if you have a family or other group who booked separately and happened to fall either side of the cut off date, I don't know, but it suggest to me that there will be nothing physically inhibiting rebooking on to one another until much later on. It may just be a case of agents having to stick to the rules, unless AA/DL have a fare more sophisticated system than I've ever used and it automatically works it out!