Luc the category id of a personal album is whatever it is in the id field. Cat id 500 is merely the members gallery which is the only hard coded catid everything else is merely what the autoincrement field is set to.
Halfhidden this thread here is about a script that was released by Zachariah originally for vb3 and an earlier vbGallery I beleive years ago. This thread has gotten in some ways way off topic because there is no script for vbulletin 4. Luciano I beleive above has posted in this thread his personal work and noted that in his initial reply that it is a very limited importer and not part of the script.