I've written a detailed tutorial for vBulletin 3 how to go about using mod_rewrite to allow your forum to have clean URLs. It goes so far as to give forum URLs names such as forum-name.html instead of forum-IDnumber.html
Dean C -- my experience is not similar to yours, but if you're successful in getting your content indexed without having to hack the urls, that's a huge bonus -- this hack is a major PITA!
cscgal -- I appreciate the work you've put into this. I SEO'd vb2 years ago, and it made a HUGE difference in traffic to my site. I've recently upgraded to vb3, and I'm putting some serious effort into properly rewriting the urls again.
I remember a while back Overgrow.com was one of the first sites I knew of to put work into SEO for vb. Overgrow opted for the archive system, which, IMO, is inferior to cleanly re-written urls. Overgrow pointed me to dbforums.com, they were one of the first sites to successfully use static urls with vb.
I'm working on inserting the thread title into the url. Also, I think duplicate pages are a bad move. A crawler should only receive one version of a page. Scripts like printthread.php and showpost.php present the same content to a bot as showthread.php. This sets you up for a duplicate content penalty. Same thing with cramming variables into hyphenated urls, like this: site.com/forum/t12345-15---2-asc--3.html and site.com/forum/t12345-10-2-10-desc--.html are sloppy, and are often duplicates. Maybe dupes like that should be excluded via robots.txt? I like threads that appear to be in the main directory -- better SEO. I think the ideal url would look like this:
Then, robots.txt could be used to exclude a crawler from any url that has variables after the .html suffix. So, no dupes.
Either way, if a variable is empty, its hyphen shouldn't be in the url. This looks bad:
site.com/t12345----10---asc-.html
Also, watch for the "." character in your .htaccess file. It's a regex wildcard, and will need to be backslash-escaped if it is to represent a period, as in \.html.
Most SEO vb hacks don't allow for styleid adjustments via the dropdown menu.
Some sites use a default style for re-written urls for non-registered members, and then feed another style to registered members. So the bots get the rewritten urls, and members get the real ones. I disagree with this -- better rankings are accorded to urls that are used, and linked to on other sites. When members are on other sites, and they link to your threads, it should be with properly rewritten urls.
mod_rewrite has a logging function that can be very useful, although it's rather verbose, even at the lower settings.
Hope this helps -- sorry for the long post. This is an extremely complex hack that most sites are not using to its full potential.