Hello there! I'm thinking of using Vanilla for a huge Mac-user-oriented forum. I have 2 questions in mind and would highly appreciate if existing users could give me some info:
1. How stable is Vanilla at high load? The resource is likely to be popular, so I was wondering if it would survive several hundred users communicating at a time?
2. How reliable is Vanilla in case someone wants to hack it?

Appreciate all the info.

Pretty much what Berg says, Vanilla is fairly stable and reliable and at the moment, there's no known security bugs or other major issues. So with that said, it's stable and reliable as can be providing we don't find another bug which would trump what I just said. The rest all falls back on the servers reliability and security.

I think 0.9.3's better for higher loads, but besides the bbcode extension I don't really know if there's a huge difference in security.

Vanilla 0.9.3 scales quite well at this point. It is also more secure when it comes to XSS attacks since I've done a lot of testing with various security people around the web during it's development. But you're only as good as the server you're on.