Our forum (not vbulletin.org but the other one I help out at) had it configured to allow moderators to hard delete, then one day someone who had been a moderator for years got mad about something and deleted a bunch of threads (which I was able to recover using a backup and a script to copy threads from one db to another). Now only admins can hard delete.

I keep that ability limited to super admins only, as defined in config. Everyone else with delete abilities, can only 'soft' delete.

Yeah I have ALWAYS disallowed anyone but a super admin to hard delete stuff. IMO its really easy to allow a soft delete and then go back and look as an admin yourself to make sure someone is not trying to pull a fast one on you. And usually I'M the ONLY admin on my site. I've gotten burned with having a second admin before as well. But some times some do not want to take the time to go through threads and look at the soft deleted threads and decide if they should be deleted or not.