Probably you can't download previous versions because you registered BSplayer only once v2.31 was already released.. I hope someone will be able to provide you a solution.
In the mean time I'll send a PM to a BSplayer team member so your topic will have more priority (I hope)
If someone with Vista 64bit has been able to successfully register BSplayer Pro (or Pro Lite) please leave here a reply and maybe a suggestion on how to perform this task