I don't know either Flarum or Vanilla, but I can tell you why using Discourse would be a bad idea in three words: Jeff Atwood's Ego. For more flavor on this, I recommend putting on your flame-retardant clothes, putting in some nose plugs, and diving into What the Daily WTF's
extensive and nasty archive of @wood's behavior during the whole beta-testing debacle.
I recommend starting at the 'Discopaedia'
thread to get a sense of just how bitter things really got between Jeff and the forum-goers.
To summarize, Atwood agreed to let Alex Papadimoulis run the testbed site for Discourse as the Daily WTF's forums. Now, Jim's whole goal of a 'civilized discourse' was immediately at odds with the tone of this forum, which could be best described as the love child of an ACM conference and 4chan's notorious /b/ channel. Jeff's reaction to the frequent, well-researched, and immensely scathing bug reports and user complaints was to deny that any of the reports were valid, and eventually to ban all WTDWTF members - and later, anyone from any other non-paying site - who had the audacity to make a bug report on discourse.com's Meta.d forum (which was used in lieu of an actual bug tracker).
Eventually, the admins had enough of both the complaints and @wood's incessant interference with their own forum, and moved to NodeBB, which is just as buggy but has developers who are willing to listen to the users.