Hi Rick, thank you very much for outlining this requirement, I'm sure it will help for any future considerations and plugin usage.
As a side note, when reviewing the VMware integration guide the plugin page, first not outline the permissions needed:
The following privileges are required for using the vCenter plugin.
As for the error code, I've created a bug to detail the permission failure in the error code.
Thanks for you reply, this was actually the information I was looking for when opening the support ticket (which actually stated "Is there a guide for setting permissions within vCenter to give some users permission to grow volumes/datastores?", but I didn't get a reference back to this document after which I started my journey of finding the correct permissions myself.