I can't confirm my doubts but I think this part of the manual page could have a wront variable for the described action:
Are you sure the &lastpost=1 is for attachments? It only act in the query at lines 297-324 into external.php (vB 3.7.2).
Code:
This option includes the first attachment of each thread as an enclosure within RSS 2.0 feeds. If the feed is requested with &lastpost=1 appended to the url then the first attachment of the last post in the thread will be included. This is the method by which Podcasting is enabled for your forum. iTunes specific settings are configures per forum in the [URL="http://www.vbulletin.com/forum/../docs/html/main/forum_manager"]Forum Manager[/URL].