When setting the subnet it requires that there be a ::0/mask in the configuration. This should allow a ::/mask in the configuration.

dhcpv6-server {
     shared-network-name facebook-v6 {
         subnet 2606:8180:0:2::/64 {  **<-- Valid Subnet address.**
             address-range {
                 start 2606:8180:0:2::100 {
                     stop 2606:8180:0:2::f:100
                 }
             }
             name-server 2001:4860:4860::8888
             name-server 2001:4860:4860::8844
         }
     }
 }

tom@vyos-rtr1# run show dhcpv6 server status
DHCPv6 Server is configured but is not running.

tom@vyos-rtr1:~$ ps aux | fgrep -i dhcp
root 3178 0.0 0.1 10036 4288 ? S Jan09 0:00 /usr/sbin/dhcpd3 -f -pf /var/run/dhcpd-unused.pid -cf /opt/vyatta/etc/dhcpd.conf -lf /config/dhcpd.leases

Now if we add the ::0/64 in the configuration everything works as expected.

subnet 2606:8180:0:2::0/64 {   **<--- added the ::0/64 here.**
    address-range {
        start 2606:8180:0:2::100 {
            stop 2606:8180:0:2::f:100
        }
    }
    name-server 2001:4860:4860::8888
    name-server 2001:4860:4860::8844
}

tom@vyos-rtr1# run show dhcpv6 server status
DHCPv6 Server is configured and is running.

tom@vyos-rtr1:~$ ps aux | fgrep -i dhcp
root 3178 0.0 0.1 10036 4288 ? S Jan09 0:00 /usr/sbin/dhcpd3 -f -pf /var/run/dhcpd-unused.pid -cf /opt/vyatta/etc/dhcpd.conf -lf /config/dhcpd.leases
root 7918 0.0 0.2 16784 11012 ? S 07:18 0:00 /usr/sbin/dhcpd3 -6 -f -pf /var/run/dhcpdv6-unused.pid -cf /opt/vyatta/etc/dhcpdv6.conf -lf /config/dhcpdv6.leases
tom 8064 0.0 0.0 6072 576 pts/0 S+ 07:24 0:00 fgrep -i dhcp