If you have multiple vyos instances in the same network/vlan and you set ipv6 ip address you will discover that sometime you can't ping ipv6 ips between some instances, if you reboot it will start work from some but from other will fail even if before restart worked.
If you run:
show ipv6 neighbors
zzzz:yyy:xxxx:1 eth0 FAILED
You get FAILED instead of the mac address of the ipv6 ip.
If you set static ipv6 mac address with the command below in the both instances the ping will start to work:
sudo ip -6 neigh add zzzz:yyy:xxxx:1 lladdr xx:xx:xx:xx:xx:xx dev eth0
Also changing the system ipv6 neighbor table-size have no effect.
I think that something is wrong sith the ndp protocol. I had the same problem from vyos version 1.2 and the last version still have this issue.
Sometime you lost ipv6 connection between ips because fail to cache the mac address.
PS: All ipv6 ips set on the interfaces are on the same /48 ipv6 network, so it work using ndp and just need mac address to communicate. I can also mention that all the time ipv4 works, even if the ipv6 do not work.