Testing again with Vault Professional 2015.... In a new test vault I placed ten (10) simple AutoCAD files into the Project Explorer under a folder called Parts. Fooled around with the Change State for a few minutes and then tried to delete all of them. But I'm presented with dialog window which says "... cannot be deleted due to vault restrictions." So I click on the Details button and click "Ignore restrictions..." but still get "You do not have adequate permissions..." message. My login account on this Vault server has full Admin rights.
I finally figured out that changing the state to Work in Progress allowed me to delete files. But I was doing testing last week and it only let me delete items if I changed the state to "Obsolete". ???
So what I seem to have discovered is that files cannot be deleted unless they are marked "Work in Progress" and Items can only be deleted if they are marked "Obsolete"... ???
Yaha by default even full admin users are denied delete rights when a file is at a Released state using the default lifecycle definitions. You can change that through the settings although I wouldn't recommend that unless you're in a test Vault.
Obviously this is dependent on the lifecycle definition you have, but typically you'll see the setting blocking the delete here:
If a file is on the BASIC RELEASE PROCESS, is RELEASED, DENY DELETE for EVERYONE.