I have been struggling to set up my Vigor 120 modem so that I can browse to the web management page to view my line stats through my DIR-655 router (655 logs in to ADSL via pppoe - sky broadband).
I have found snippets of information and information from Draytek support directly that has led me to change my setup as follows:

Change ip of vigor120 to same range as local network and turn off its dhcp.
Connect rj45 from 120 to switch and two further rj45s from switch - one to wan and other to lan port of dir655.

According to Draytek support this should be working and I should be able to browse to the 120s web interface at the ip specified within my network. However, for me, it does not - cannot ping the 120s ip or browse to it - the lan lights on all the network devices (router, switch, modem) permanently flicker frantically and PCs connected to the router lose internet access. Only resolves by disconnecting the rj45 from the routers lan port, but then I am back to square one!

My PC's all connect wirelessly to the dir-655, in case that makes any difference.

Draytek are simply saying that it should work......
Unfortunately, as I am no network expert, I have no idea what all the above symptoms mean or what to do next.

Any help, thoughts, opinions would be most welcome.

Anybody got any advice on this?

How do others with vigor120s get their line stats?

With that setup, you need to turn the DHCP server off on the Vigor 120 and change its IP address so that its on the same subnet as your DIR655 router, with an IP that wont conflict with anything.
Then connect it to the LAN side and run a cable from the LAN side of your DIR655's network to the WAN port on the router.
This only works because PPPoE doesn't use IP addressing so it won't cause any conflicts but you'll be able to get to the web interface of the 120 from the LAN and it'll still establish the PPPoE connection (should do)

Surely if anyone on the lan side decided to set up a pppoe connection on their desktop, this would cause issues in this "looped back" configuration??

It probably would, but the 120 is unlikely to respond as it should already have an established connection with the 655. It's really not a perfect way of accessing it, since it wouldn't work with dual PPPoE WAN connections without some careful VLANing, but it is the only way that I know of to access the modem when using PPPoE.

@voodle, did you look into using a static route instead during your investigations?