In this setup, it would seem I can only use one of the WAN links at once i.e. the prefferred one, and the second link seems dormant until it is needed (that is, the VDSL goes down). However, I would like the secondary connection to be available too, so both connections are in an active/active state. There are a couple of reasons for this:-
1) I'd like some traffic to go out via the VDSL connection and other traffic via the ADSL connection
2) I have rules setup against the static IP address of the ADSL WAN connection, which although I could reconfigure, I would like to remain as they are, but at present because this connection seems to only work when the VDSL goes down, it's not workable.
Re: How can I use 2 WAN (ADSL+VDSL) links at once?
You can use policy based forwarding to have some traffic go to each ISP based on filter match criteria. this can be source addresses or ports to split the load. This kb article shows how to set it up sending one ip range to each of two ISP.