This is just a guess but with LXD v 3.0.0 there is a new tool called lxd-p2c which will convert a physical machine or a VM to an LXD container. Read through this thread to learn more about it:
My thought was perhaps you could create a VM with Vyos or FreePBX then use lxd-p2c to create your initial LXD container.
You may still have quite a bit of configuration to do regarding networking interfaces (vyos) or whatever devices FreePBX might use but at least you would have a starting place. If you can get everything working in the initial container you can then always copy that container either local or remotely to replicate?
With FreePBX this thread on the FreePBX forum seems to indicate that it at least worked okay in a “privileged” container but from the thread discussion it sounded like the OP of that thread didn’t have a lot of experience with lxc yet so perhaps it would work in an “unprivileged” container also?