dhcp6c seems very old: 20080615-24 https://manpages.debian.org/testing/wide-dhcpv6-client/dhcp6c.8.en.html

other projects seems to actively patch dhcp6c:
https://github.com/opnsense/dhcp6c

some have completely rewritten it:
https://github.com/openwrt/odhcp6c
https://wiki.archlinux.org/title/Dhcpcd

debian is also pushing for systemd-netword handling:
https://wiki.debian.org/IPv6PrefixDelegation

Is there any plan to replace dhcp6c on vyos ?