In my case, the container is created and running using IPv4 only. The network it is in has a defined prefix for IPv4 and IPv6. Then, the only thing I try to do is add an IPv6 address to the container. The network it is connected to already has the IPv6 prefix defined. That is when it dies.

Yes, if I remove the container, and then as root create the container using the exact same podman command that VyOS had used, but just add the --ip6 definition, then the container is created and responds on that ip.