Vanilla 1 is no longer supported or maintained. If you need a copy, you can get it here
I found a previous discussion  which started out addressing this issue, but in the end seemed to settle on emails notifying of an update. What I'm looking for, and what some of the comments in that discussion discussed is the ability to post a comment, or even start a new thread via email. That means never using the browser or visiting the site. Everything is done via an email client the same way as mailing lists. The forum would simply serve as the archive for the list and a secondary way for users to post comments etc. That way the 'old-school' people feel like they're using a list and the 'new-school' (??) people use the forum and the difference is transparent to each set of users.
Personally, I started off using forums and have went 'backward' to lists, so when a newbie to a list asks for a forum and the seasoned users resist (unless they can post via email - see they are not totally unreasonable), I understand. Then again, I suppose all the crud in most forum packages has prejudiced them (as it did me - well, at least that's mostly what scared me away). With such a nice clean system like Vanilla and the option to not change their work patterns (works like a list) some of those people (myself included) might be more prone to adapt. And then, after they come to the forum to edit a few errant emails they fired off to the list/forum, they might even make a few more concessions.
Interestingly, I found a plugin  for phpBB  that does just this. Of course, I have no desire to use that forum package for the same reasons I like Vanilla. Although I did note that expanding the plugin to work with other forum packages was listed as one of the planned additions to the project. Maybe someone could look into using their code as a base for an extension.
I'd tackle the thing myself, but my time is already spread pretty thin and I haven't played with Vanilla as of yet (although I've read some of the docs and it looks promising - I think I might just borrow from the Object Factory concept in my own work - brilliant). Any other takers?