UPDATE JUNE 17
The site is currently on version 4.2.5. vBulletin is now at 6.1.2 for stable release.
There is NO upgrade path that avoids breaking functionality of the site. It was left at 4.2.5 too long.
All of the current plugins have no upgrade path. So, we are going to get the core code to version 6.1.2 stable and then start adding features and functionality back in over time.
EXPECTED DOWNTIME
vBulletin is handling this upgrade with us. They estimate 72 hours of the site being offline as a minimum, but it could go longer if they find unexpected things like code hacks, and custom work arounds.
The good news is if things go completely sideways, we can restore the snapshot backup to the time just before they started and at least end up back here with a (semi) functioning community.
I will update again with dates for this outage once we have that info from vBulletin.