genuinely if you dont see the vault store groups in ev8 then you need to roll back immediately regardless of newly archived users etc
there simply isnt any other choice

you may have to disable and re-enable newly enabled users since then, but you have to roll back the entire environment and then move forward

So, everything that was updated since Saturday would have to be rebuilt/redone?

Not sure if this will work in your case seeing as you went through an upgrade

If since you ungraded the first time and you had restarted archiving after the upgrade. There might be the possibility that if you have your truncated SQL logs since then that you could replay the logs and repopulate the database. 

Seeing as the archived files are still there and the database could possible be replayed to repopulate the data then you could update your indexes and get back on track

Note that I have never tried this in a situation like yours. There was issues with your upgrade and I'm not sure how it will affect the replay seeing as some aspects of the upgrade failed to complete. But if the database was successful in the upgrade and the data archived after your first upgrade is good then in theory this may work but you will need to test it in a LAB environment first to ensure it does not screw up your environment

I have done this for a SQL failure and it works great but as I said you need to test it in a LAB first