As with other routers, Vilfo's VPN router works fine together with different Fritz!Box models. There are a few things to think about though.
You will most likely not be able to replace the Fritz!Box router with Vilfo, but you can connect Vilfo to the Fritzbox. To do so, connect a cable from your Fritz!box's LAN port into Vilfo's WAN port.
An IP collision will most likely occur as your Fritz!box product uses the same default IP span as Vilfo's default values. Unfortunately it is not possible to bridge Fritz!box modems, so follow this guide to resolve the IP conflict so you can use Vilfo's VPN router together with your Fritz!Box product.