Cannot attribute both IPv4 and IPv6 to a VLAN interface

[Ryan Stone]

It seems to me that the mistake that you are making is that you are
setting the vlan portion (e.g. vlan 18 vlandev em0) twice.  The second
time that the configuration is applied it fails.  I believe that you
want instead to put the following in rc.conf:

ipv6_ifconfig_vlan18="xxx:xxx:xxx:460:: prefixlen 64"
ipv6_ifconfig_vlan19="xxx:xxx:xxx:461:: prefixlen 64"