Append these to /etc/network/interfaces:
iface vmbr0 inet6 auto
up sysctl -w net.ipv6.conf.vmbr0.accept_ra=2
up sysctl -w net.ipv6.conf.default.accept_ra=2
up sysctl -w net.ipv6.conf.all.accept_ra=2
up ip token set ::3/64 dev vmbr0