Try 3.6.3 and see if that helps, it has a different VDSL modem code, from ftp://ftp.draytek.com

If that doesn't help, put it back on the current firmware, get this VDSL information after its synced but isn't passing data:
http://www.draytek.com/index.php?option=com_k2&view=item&id=2068:faq-article-2068&Itemid=296&lang=en

Ideally if possible, get a packet capture using wireshark from the point that the VDSL is plugged in up to the point that the PPP establishes using this method to capture packets from the VDSL:
http://www.draytek.com/index.php?option=com_k2&view=item&id=2069:faq-article-2069&Itemid=296&lang=en

Then give that to draytek support

Hi.
Out of interest- can you go to the VDSL status page when it's connected and post a screen shot of your VDSL connection details?

PS Found this: http://blog.gudge.com/2011/11/pppoe-draytek-2820n-vdsl.html