The Vodafone router is a one box solution. You can't use it as just a modem nor can you use it as a router with an ethernet connection to a separate modem.
There are threads about using different equipment on the Vodafone forum. I have a Zyxel VMG8924-B10A which I plan to use. It has better Wifi than the Vodafone router and will be good enough for me. There are better Wifi products if that is important to you.
You should not encounter any problems getting your userid and password from Vodafone. If asked what equipment you will use you need to specify something on the BT SIN 498 approved list which cab be found here
The userid is in the format dsl000999999.@broadband.vodafone.co.uk and the password is 8 characters.