This is the first I have heard of Vanilla, but that isn't going to stop me from critiquing it.
First of all, Vanilla may have been around for a long time, I don't know, but even that doesn't mean it has been properly vetted. The community needs to put it through its paces.
There are many many forum applications already. What does Vanilla bring to the table that others haven't already? I can't tell from their home page, which, IMNSHO, means that even they don't know. Anybody with at least a minuscule amount of marketing knowhow knows that you need to tell visitors immediately what your differentiator is. They talk about being simple to use, but a) that is relative to each individual's technical savvy and b) not a big deal as even mom & pop hosting services now have push-button installs of forum software on demand.
They claim add-ins/plugins/mods are easy to do. Yeah, until you need to worry about deconflicting two different community-built ones. The first mod is always easy.
Vanilla may turn out to be wonderful. I don't know yet. And that is why you posted - looking for others that have used it and can comment on it. However, I just thought it would be a good time to point out some generic things that may be useful when looking at any software of this nature.
And one last comment. From their home page,
Vanilla is all about add-ons. We've spent many, many months developing a new system whereby developers can create add-ons for Vanilla that allow you to do just about anything with your forum. Want to turn your Vanilla into a bug tracking system? Want to tag discussions instead of categorizing them? Maybe you just want a fancy highlighting effect when new messages are posted. Your imagination is the limit.
Unless you are building a framework, anything that tries to be everything to everybody tends to not doing anything well.