I feel your pain. Until recently I was the owner of a vBulletin based community with over 31,000 members and almost 4 million posts. Due to it's size and customisations we didn't upgrade past 3.6.X. We ran vB 5 on our test site and hated it. I've always been a vB guy so I stuck with it in the hope of improvements but things went from bad to worse.. it was harder to maintain, the site looked dated, the tempting system is awful, though luckily we didn't have any hacks to worry about.
We tried 'integrating' modern/social tech but it always felt plugged in as appose to being a natural part of the site and every day we were losing members to social media.
I decided to part ways earlier this year as I didn't have the time to modernise the site.
I think ditching away from vB is a smart move. I am a member of a community that uses IPS and it is light years ahead of vB. I can't speak for administration tasks but definitely a better user experience. One thing that does stand out is that it feels quite 'heavy'.
Whenever we need forums now we use Xenforo, and I advise you to try the demo and consider it. Most communities I visit have migrated to Xenforo and with good reason... It is very easy to style, customise and has modern social aspects. Most importantly, it's lightening fast.
Good luck with the migration - I'm sure you have a great team behind you, however if you are ever looking towards the community to pitch in I would be happy to do so.
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