OPNSense does maintain a version of wide-dhcpv6-client, but as far as I know, neither Debian or any other linux distribution use this repository as their upstream repository.
May 19 2020
Here is an expanded concept covering a few more areas of the config. It's not as pretty as I'd like, but doing it this way allows for an early translation pass and the result would be a config more or less with everything how it would be if it had been hand entered in VyOS as-is. Tying things to the sla-id also makes consistency checking simpler, can't add a prefix-sla-id block unless a delegation defining it exists, and can't delete the delegation until nothing references its sla-id any longer. I feel this can probably be improved upon, suggestions and criticisms welcome.
May 18 2020
I don't have much preference configuration-wise between the isc and wide clients, but wide-dhcp6-client is not actively developed and is less rfc compliant than dhclient. Not deal breakers for my needs, but the existing vyos scripts already assumed dhclient so I figured it would be a little less work to get things properly integrated once I learned my way around the scripts and even if my changes were not adopted, it would have at least made it easier to keep things synced as time went on.
My take at the client config following the config I had:
I had made a custom image with wide-dhcpv6-client a week ago or so, this is the client config I was running for NA and PD, WAN is eth0 and gets assigned the NA address with the prefix assigned to eth1 with the address <prefix/sla-id>::1 (if ifid is not set it will assign EUI-64 address). The lifetime value was pulled from an example config and I left it as-is while testing.