For posts in a thread, a record of the last post read in the thread, by the user, would have had to been stored. Looking at the database tables I do not see anything that can be used to derive it and unfortunately the way vb handles unread posts does not get us there either. It might be possible to extract recent read post information from the activity stream, but I have to ponder that some more when I have a fresh brain.
Sorry, for the bad news, it would have been nice if it was easily possible.