The version/build of vCenter is 5.1.0 Build 1123961. I have tried a local admin, and since it's SSO I also tested admin@System-Domain (after granting it proper permissions in vCenter) both with identical results. The circled line is the account I'm using in this screenshot of permissions on the vCenter server itself:
Unfortunately there would be no way to setup another vCenter and the environment is too large to deploy a VCVA (even if the sizing has changed, SQL is the database backend so I'd still need a Windows based vCenter install at the moment). As for reinstalling the plugin, I have done that, as well as redeploying Orchestrator - I suppose I should have mentioned these things, sorry. 
I tend to only ask if I'm really stumped on something. 
On that note, it may be that I need to open a support case, I just wanted to make sure I wasn't missing something silly first, plus these discussions may help others and are excellent for tracking the issue easily.
~Brandit