I noticed the issue trying to assign an dhcpc6 after the work on T2265. Looking back at the code, the issue most likely exists before my involvement with VyOS.
I noticed that the vyatta-dhclient-hook was not v6 aware wand would overwrite the v4 ".leases" file.
I am not sure if this is really a problem, as I believe it may not be used by DHCP itself.
I have a patch part of T2265 included
When running the program manually with "-d" the following "helpful" advice is provided:
root@vyos:/var/lib/dhcp# /sbin/dhclient -6 -nw -cf /var/lib/dhcp/dhclient_eth0.v6conf -pf /var/lib/dhcp/dhclient_eth0.v6pid -lf /var/lib/dhcp/dhclient_eth0.v6leases eth0 -d Internet Systems Consortium DHCP Client 4.4.1 Copyright 2004-2018 Internet Systems Consortium. All rights reserved. For info, please visit https://www.isc.org/software/dhcp/ no link-local IPv6 address for eth0 If you think you have received this message due to a bug rather than a configuration issue please read the section on submitting bugs on either our web page at www.isc.org or in the README file before submitting a bug. These pages explain the proper process and the information we find helpful for debugging. exiting.
I found this help page from ISC: https://kb.isc.org/docs/aa-00368
As I have limited experience with setup up DHCP .. I stopped the investigation there in hope someone else can help.