Using a Vigor modem to connect a 'cable' router to an ADSL connection (and solving the PPPoA vs PPPoE issue at the same time) makes complete sense. I can't see what you're expecting to gain by replacing BT's own modem for VDSL.
I considered them when I saw SEG launch them in the UK but the high price put me off. I've sourced a number of new Huawei VDSL modems off The Bay instead...