These two threads:

http://bbs.scoobynet.com/photography/626729-printing-advice-canon-400d-photoshop-7-canon-pixma-ip4300-bad-print-print.html

http://bbs.scoobynet.com/photography/626725-printing-advice-canon-400d-photoshop-7-canon-pixma-ip4300-bad-print-print.html

They each come out as a single browsed append without all the normal forum layout, in both FF and IE.
Maybe something in the thread titles is messing up the forum software ?

I'd say you're pretty much spot with the problem being down to the thread title.
If you view a normal thread, you have the thread tool option of "Show Printable Version", when this is selected the url of the thread has "-print" appended to the end of it. The url for the threads you mention would both normally end in "-print" as they are search engine friendly. My guess is that when you click on the thread vBulletin is reading the last, pre extension part of the url, seeing that it's "print" and is deciding that it's meant to display a print friendly version of the page.

I've had a quick search and found the following info on the vbeso forum:

you should modify your "'Printthread' URL Format" in vbseocp to:
[forum_title]/print-[thread_id]-[thread_title].html


This would put the "print" part of the url at the start of the url, rather than at the end, thus preventing a problem.