Yes, I think this can be done. And yes, while it would be nice to have it supported in the PHP code that runs vBulletin, I think you can set this up yourself, especially if you are the tinkering type.

First, you have to be willing to use 3 programs / extensions:
1. Firefox (web browser)
2. GreaseMonkey (extension for Firefox)
3. AutoHotKey (key macro / remapping program)

(Note, #3 may not be necessary for you... I forget how the IBM thinkpad back/forward buttons are mapped - you may be able to do this directly in your ibm software. However, you may find it useful for other uses.)

The quick rundown...
Use Firefox because it supports GreaseMonkey. I think I heard something about an IE-compatible something that did similar things, but I haven't looked into it myself.

GreaseMonkey lets you create/install a script that will re-write the code of webpages, usually to add extra information, change the layout, or provide added functionality.

What you may not have known is that there is HTML code that lets you assign "access keys" to links. Try it now... press ctrl-3 right now (maybe alt-3?). It should bring up the printable view of this thread. There are a number of these access keys programmed in to the vBulletin pages... alas, not for the next thread link, though.

You will want to use greasemonkey to rewrite the page to add in the code to assign an accesskey for the next thread. The HTML code looks like this:

<a href="linkhere.php?parameters" accesskey="r">
will change the link for linkhere.php to have an access key of ctrl-r.

Use greasemonkey to find the Next Thread link, add the access key code, and voila. Keyboard access to the next thread.

Then, either use AutoHotKey or your keyboard software to remap the key combination or button to the new accesskey you setup, and you now have your forward button for the next thread.