Questions...

• How much control do you have over what the exact url is? Could you make something like yoursite.com/forums/[board-name]/[thread-id]/[thread-name]/[thread page]?
• Would it be possible to make a thread url that didn't include its ID anywhere, instead adding a number on the end if there was more than one topic with the same title?
• Could you edit the exact url on a per-forum basis, or even per-thread?
• Can you exclude certain words from what goes in the url? For example, "The" or "An"... I know one of the free SEO scripts on vBulletin.org does this.
• I'm currently using vbDrupal, but I'm assuming it'd work the same with any CMS-type script for vBulletin... are there any problems with displaying the new urls with them? Would lists like "Last 10 posts" and "top 10 posters" link to the rewritten vBSEO urls, or the old ones? If they'd link to the old ones, how hard would it be to fix?
• This is related to SEO in general more than vBSEO specifically, but the majority of vBSEO forums I've seen seem to have ".html" on the end of their URLs. I think having the content name without any extension on the end would fit the feel of my site better, but is that known to affect SEO ratings at all? Would .html be better?

1) Quite a bit. You can play with your own virtual instance of vBSEO on our vBSEO demo: www.vbseo.com/demo
2) No, this is not possible.
3) No, this is not possible.
4) Yes, vBSEO has a stopword list. (You can see that in the demo also.)
5) You can use vBSEO's Functions for Extensibility to rewrite URLs on non-vBulletin pages.
6) I don't think anyone knows the answer to this for sure. You can see that the format here on vBSEO.com is the way you mention. The format used here is included as one of the pre-set selections in the vBSEO control panel. (You can test this in the demo also.)