Hi,
I haven't tested vsphere plug-in with SnapCenter 4.2, but looking at the SnapCenter 4.2 documentation, it appears that a new 'broker' application is needed, have you got this installed.
https://library.netapp.com/ecm/ecm_download_file/ECMLP2858035
In case Data broker is already installed, then take a look at this thread:
https://community.netapp.com/t5/Data-Backup-and-Recovery-Discussions/Data-Broker-1-0-cannot-register-SnapCenter-Plug-in-for-vm-with-SnapCenter-4-2/td-p/150670