It is the publisher that determines if there will be any pre-order bonuses, or collector's editions, etc. I assume with the North American Xbox release being delayed compared to everywhere else, and the PC version being available for awhile, that a little extra something would help sales, or attract attention, etc.
Larian may (or may not) add additional items to the Vault at some point, though presumably there is some time frame where the soundtrack and art book will be exclusive to the Xbox pre-orders.
Promotional gubbins e.g. lithographs etc
