Just a guess:
If your Vizio smartTV at some stage would have been part of your router's home.arpa before you moved it to private, your router's DHCP server may hold on to the MAC-to-name association it first learned (under certain circumstances, Pi-hole's DHCP server may do something similar).
If that would have been the case, explicitly deleting a potentially still existing DHCP lease/reservation/static IP for the device from your router may also remove the unwanted name. If such a lease doesn't show up anywhere in your router, or your router doesn't support editing or deleting such an entry, then you may still get lucky: Try reconfiguring your DHCP name server for a slightly different DHCP range (e.g 192.168.1.21 through 192.168.1.254 if it currently would be 192.168.1.20 through 192.168.1.250). Often, this would force your DHCP server to forget any previously stored information and populate its respective internal storage afresh.
If that doesn't work, creating local DNS records within Pi-hole is still easy enough, and it should also suffice to address your issue.
If you don't trust your router's configuration anyway, you may just as well start with creating them. 