I contacted my VPN provider (Express VPN) and they said that they cannot provide me the IP of their DNS servers. I know if I use their app (which has a RPi version) that it will do this but then DNS is set automatically but then the problem is that I still don't know the address to hardcode it into Pi-hole. At a loss here.
What I was thinking was possibly making it so that the Pi never uses Pi-hole for any of its own queries. Instead, only clients from the LAN use the Pi-hole service. If I can figure out how to turn off Pi-hole for the Pi hosting it then this would resolve this issue.
Sure anyone could see the traffic the Pi using but as a router the only time the Pi uses the WAN connection for itself is when it is updating packages.
It does not. I know that I can push the Pi as a DNS to its clients but I have chosen not to. Not all clients on my LAN use the Pi as a gateway so I have left my router as the DHCP server and manually set both the default gateway and DNS to that of my Pi if I wish for it to use the Pi tunnel.