A device does not register its name directly with DNS.
A device may send its hostname to a DHCP server during DHCP lease negotiation. It then depends on the DHCP server whether that would register the requested hostname with a DNS server, e.g. by default dnmsasq would do so, as would pihole-FTL if Pi-hole's DHCP server is enabled, but ISC DHCP server wouldn't .
Pi-hole will read hostnames according to its dnsmasq configuration.
By default, it will read /etc/hosts, /etc/pihole/local.list and /etc/pihole/custom.list.
If Pi-hole's DHCP sever is active, it may store hostnames in 04-pihole-static-dhcp.conf, and it may also consider information from its DHCP leases /etc/pihole/dhcp.leases.
Pi-hole will also use any custom hostname configurations, like host-record, cname, addn-hosts definitions etc. that appear in its dnsmasq configuration - in your case, there's 02-pivpn.conf pointing to an extra hosts file.
Pi-hole will also try to source hostnames of its local clients via reverse lookups regularly (once per hour), either through its regular upstreams or through its Conditional Forwarding target IP address, if configured accordingly.
It may store names learned that way in its database.
You may control how it does that via a set of pihole-FTL.conf options (NAMES_FROM_NETDB, REFRESH_HOSTNAMES, MAXNETAGE).
But before you start playing with those options:
When we checked this last for your Vizio TV device, your router has been the source of the hostnames.
You should once again verify what your router would return for the hostname in question, by forcing normal and reverse lookups to your router's respective IP addresses, e.g. for a device named mini at 10.10.0.51 and yor router serving to subnets at 192.168.1.1 and 10.10.0.1:
nslookup mini 192.168.1.1
nslookup mini 10.10.0.1
nslookup 10.10.0.51 192.168.1.1
nslookup 10.10.0.51 10.10.0.1
If your router is the source of those unwanted hostnames, my previous advce would still apply: