I wrote some code that inserts a new post into a forum when someone posts a classified (in fact, it posts in a respective forum depending on which type of ad is placed); however, the code is for .threads.
I saw a post on vBulletin.Org about how to insert a post automatically, so it shouldbe possible to take my code and modify it to insert a new post into vBulletin. It's just a section of code that goes into the uploadproduct.php script. If you have some PHP experience, I'd be glad to try and help you through it so we can post the results.
Please do not PM me for support or sales questions. Thank you for your understanding.