I am trying to setup a Vigor 120 as a PPPoE pass-through modem with a Sonicwall TZ400 configured for the PPoE, the broadband connection I have is with BT, however I cannot seem to get it to connect properly and was wondering if there are any settings I need to tweak on the Vigor 120 for this to work. for example should I change the modulation to ADSL2+ G.992.5 or should multimode work?

You shouldn't need to change any settings.
Try setting it up from a PC or at least confirming it can get sync.
You may need to hardset the IP address of the PC...I can't remember if the V120 does DHCP.