That's an interesting question, @ChallaPradyumna. Especially because the real benefits of both vB and IPB are often not visible to many users of the platforms.
Asides from all the PR (marketing around features, which often is the result of the age of the platforms), the true strength of both is that they can deal with very large communities. Sure, server load is server load and hosting requires upgrading (or more cloud computing units) as forums grow, but both vB and IPB truly grew in strength back in the day when MSNBot would literally DDoS pretty much any forum with several 100k posts/threads.
If you go back to 2002-2003 this was truly the horror scenario for many forum webmasters. The necessity to start optimizing URLs (especially for Google), combined with the serverload both MSNBot and GoogleBot would cause back in those days. The monthly MSNbot visit was evil, but a necessary evil.
In those days vB and IPB especially came to the fore as solutions which had those covered and also offered 'archive' versions. Bandwidth was still a thing then and the true strength of IPB and vB were (given decent enough hosting obviously) their speed, capacity to deal with large number of users, caching, and search query return.
And their preparedness for searchcrawlers. vB was also one of the first (affordable) forum solutions to offer CMS functionality (you could build a portal and forum posts could become 'news articles', almost like blogs - the Slashdot website still is rather reminiscent of how 'forum portals' would look bad then, complete with threaded comments shown as 're:' links).
One of the easiest open source solutions to similar functionality then was PostNuke, which was pretty much 'CMS/portal with easy phpBB forum integration - simplified, don't beat me up for this large misrepresentation due to simplification). But security and speed, scaling was what Invision and vB had focused most on. Plus the whole forum administration kitchen sink.
Invision since has evolved more along the path 'we've thrown every possible kitchen sink we could find' in the feature list and additionally we are even serving it all from pay-for-your-needs cloud hosting.
vB Has come very much to a standstill and still sell the old 'try hacking our software' as main feature and unless configured wrongly, vB does well in that aspect. Although I would still recommend to include Akismet or any other decent anti-spam solution.
Synopsis
Way too many words if I could have said things as simple as: So far I doubt any Flarum setup has reached a point yet where the true differentiators of vB and Invision can be compared (administration/maintenance kitchen sink, speed for very large forums, caching).
All the rest... many of their 'other features' you won't miss them unless you're a die-hard BB forum rat and not open for change. True usability of both is achieved thanks to... extensions.
Disclosure: the fully fledged CMS, social network, and then some Invision is difficult to compare to Flarum. One's forum software, the other is 'hey, we could also add that'. ?