I just had a member complain to me that his private vBulletin galleries became public after the import to XenGallery. I can't really provide any reproduction steps aside from trying to import vBulletin 3.8.
If you still have access to the vBulletin database, could you please run the following query:
SELECT albumid, state
WHERE albumid = 123
With this query you will need to change vb_ with whatever your vBulletin database prefix is (or remove it if you don't have one) and replace 123 with whatever the albumid is for that album (in the VB database).