Configuring VPN behind firewall

Looking for advice on this: Looking at setting up a VPN for site to site purposes. The current setup is ISP Router > Firewall > Internal Router

Both Firewall and Internal Router have firewall capabilities, wondering which option would be best to set the VPN on, Firewall or Internal router, or if there is such thing as 'best' option. I was thinking about the internal router, but would it be possible to set it up on the internal router given the current configuration?