If you’re thinking about using Discourse - just use it! It’s the best forum platform around and will be for the foreseeable future. I’m often staggered by the pace of development here, and the team is made up of some incredibly talented people (such as Sam Saffron who I’d say is one of the most experienced Ruby developers around).
With regards to what’s missing from vB - that’s probably not a good way to look at it because they are different, but I am pretty sure that if your site is predominantly a forum your users will be much happier on Discourse.
The only ‘forum’ type feature I miss from vB is the ability to have a section where only the topic starter and the admin/mod team can view threads. We use these sections for Forum Feedback, Contact Staff etc and they’re just a bit better suited to that type of thing than PMs (which once seen are forgotten and not as easy to categorise or review later).
There are lots of things I’d like to see in Discourse tho - one of them is being able to use it to power lots of other sections, such as blogs, articles, news, etc. The commenting/discussion system in Discourse is probably one of its biggest strengths - imagine a Wordpress type blog with the power of Discourse comments and all seamlessly integrated with the forum’s user profiles. I really think Discourse could replace Wordpress (and other CMSes) if they looked into this and it really wouldn’t be too difficult either - on vB we used it to power several other sections and all we needed was 1) A custom index page for each section where we could fetch all the latest items for that section, and b) the ability to set one style for the first post, and another for subsequent posts (simple conditional managed this in vB) this would allow the main article and comments to look different.
I’d also like to see a proper block feature. This would make life a lot easier for moderators.
Both of these have been discussed on the forum before, but they are not likely to be added (which I personally feel is a big shame and I hope the team review this at some point).