In order to post more than one attachment to a thread, you need to reply to it, and in that new post, put another attachment. One attachment per post is the limit. But, any number of posts can be put into a thread, each with it's own attachment.
When we update to vbulletin 3 upon it's final release, there will be an option to upload multiple attachments in a single message along with being able to manage the attachments.