Can the Asus ZenWiFi AX router be connected to existing VDSL service

That leaves you in what's called double NAT. It still works but I don't recommend it because you have two routers in the mix.

At the moment, the Gigabox establishes the internet connection and receives a public IP from Vodafone. It then routes that traffic to your private network. If you bridge the Gigabox, it just acts as a passthrough and allows your router to establish the connection. If you don't bridge it, the Gigabox still establishes the connection and routes the traffic to the private network, but then your router does the same thing again. It can cause problems with applications or devices that expect direct connections, such as game consoles.