Ok last step in my decision.

[gavinzac]

you need to print out both the joomla articles table structure and the vbulletin post structure.
draw lines matching corresponding field types (eg article.text -> post.text, article.author -> post.username), come up with default values for any remaining fields, and then copy from the joomla to vbulletin.

ive done this to convert joomla articles to phpbb so vbulletin shouldnt be any more difficult.