This seems like one of those "instructions not clear moments" for me.
VSC 7 removes backup and recovery so at this time you have to manually migrate your backup jobs into SnapCenter which is fine I'm doing this in our lab to prep for production and understand the mechanics.
I am unable to get the vsphere plugin to install with the vcenter virtual appliance. What credentials should I be using? I should be targeting the vcenter server appliance for plugin installation right?
When I use root credentials it says invalid credentials, and when I use vcenter server admin credentials it says host not found, and when I use a domain account that has administrator roles in vcenter it says specify a valid run as account or verify credentials.
What are the proper steps to register the snapcenter plugin with vcenter?
There are a few things here that seem mixed together, so I am not 100% sure what you are trying to do.
Generally speaking SnapCenter 2.0 needs to use VSC 6.x for the purposes or protecting virtualized databases and protecting VMware VMs and/or Datastores.
SnapCenter 2.0 doesn't have any native VMware virtual machine or datastore backup and recovery features, so the backup and recovery features of VSC are used.
SnapCenter 3.0 does not have any direct integration with VSC. This is becuase with SnapCenter 3.0 native capabilities for VMware virtual machine or datastore backup and recovery features have been added. VSC integration is no longer needed. In fact VSC 7.0 no longer has backup and recovery features as SnapCenter has completely taken them over.
SnapCenter 2.0 can not be used with VSC 7.0. There is no integration there.
SnapCenter 3.0 and the SnapCenter Plug-in for VMware vSphere needs to be used instead.
Concerning installing the plug-in, just like VSC, SnapCenter requires a Windows host to install the plug-in software.
This should be a standalone Windows host (physical or virtual, doesn't matter), but not the SnapCenter Server and not the vCenter appliance.
SnapCenter Server <--> Windows host for vSphere plug-in <--> vCenter <--> ESX
When you install the vSphere plug-in it is pushed out from SnapCenter and installed to the Windows host. Then the Windows host connects to vCenter.
The run as account that you specify when installing needs to be a domain user with local administrator access.
I am using VSC 6.2.1 to perform backups (and update SnapMirror and SnapVault destinations) in my vSphere 6.5 environment.
I plan to upgrade from ONTAP 9.1P8 to 9.3, which requires VSC 7.1. This facilitiates moving to SnapCenter for the backups. The documentation is not clear on how you actually upgrade and how you retain visibility of the VSC 6.2.1 backups so you can continue to run backups and restore the older backups that VSC created.