@mentalinc @taubin doing that will result in a broken IPv6 configuration. Please run "delete interfaces ethernet eth0 ipv6" then commit and save. Your /etc/radvd.conf file won't be there but I'll do some investigation and post.
I did fix it with my own router and there is no IPv6 based configuration directly under the ethernet interfaces. Long story short it is a bug in "vyatta_gen_radvd.pl" in /opt/vyatta/sbin that is causing a bailout if an older IPv6 prefix is not set however this breaks DHCPv6-PD if you set this (hence why your IPv6 won't be working) - this bug has existed forever and is yet to be fully fixed...