Ok so the way I've configured it to work is:
Draytek WAN 192.168.2.2 to Vodafone LAN 192.168.2.1
Draytek internal DHCP for LAN and WLAN is 192.168.1.x
Vodafone router - DMZ'd the Draytek IP (you can set only one IP to be DMZ)
This works fine but I have a complication on the line.
Draytek has a LAN to LAN IPSEC VPN going outside to a provider.
The problem I have is the VPN tunnel drops like clockwork every 20 minutes when it sends DNS request for the name of the VPN server.
I found this out checking my PiHole logs.
So I thought that the DNS entry on the WAN port config is the key here, I have changed it to an external DNS (Couldflare) thinking the router mixed itself up in queries over WAN to the LAN Pihole.
Vodafone was also reporting System (not Firewall) errors with some requests from various IP's to Draytek but I checked some of them and they were dropped as scanning.
The problem persists though and the connection keeps dropping.

So I went back to the old setup of Draytek doing the ADSL dial and VPN out via the ADSL and the tunnel does not drop.
Not sure how to deal with the tunnel problem but my original problem for using Vodafone was the Draytek loosing sync on the ADSL periodically, I have changed the modem code option and so far it's been more stable and the VPN via ADSL with no Vodafone router in the way is solid as well.
Time will tell