The Internet Archive does what they can, but there's no guarantee that they'll exist a decade or two from now. In that light, I try to keep personal backups of sites that I might want to look back on twenty or thirty years from now.
I'm unfamiliar with the vBulletin backend - how difficult would it be to make a tar of the public-facing part of the website? Essentially, a dump of everything that can be seen as a guest on the forum - stripping out PMs, user information, and so on.
If that's unfeasible, a script to archive specific threads would take care of most of what I care about; I think Calibre has a plugin that can grab from Xenforo - I could look into seeing if it can be made to work with vBulletin.