https://github.com/zevlag/pfsense-ports/tree/master/pfSense-pkg-zerotier
https://docs.opnsense.org/manual/how-tos/zerotier.html

pfSense and OPNsense are getting there.

https://github.com/openwrt/packages/tree/master/net/zerotier ZeroTier is now an OpenWrt package.

I posted a comment asking Adam over at ZeroTier to see if he could help with this task.
https://github.com/zerotier/ZeroTierOne/issues/418

Hmm no movement...

ZeroTier supports Debian. And VyOS is based on Debian.

So I dropped to a shell in VyOS via sudo su and installed ZeroTier on VyOS 999.201706282137. Now it shows an IP addressed ZeroTier interface in iproute2's ip address but if you jump back into the VyOS console show interfaces does not list the new interface. Interesting...

Upon reboot the zt interface is still shown in iproute2 but not in vyos shell. My first thought is it's failing somewhere in here:
https://github.com/vyos/vyatta-cfg-system/blob/current/scripts/system/vyatta_interface_rescan

root@vyos-beta:/home/vyos# ip address | grep zt
4: ztREDACTED: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 2800 qdisc pfifo_fast state UNKNOWN group default qlen 1000
    inet 192.168.23.2/24 brd 192.168.23.255 scope global ztREDACTED
root@vyos-beta:/home/vyos# exit
exit
vyos@vyos-beta:~$ show interfaces
Codes: S - State, L - Link, u - Up, D - Down, A - Admin Down
Interface        IP Address                        S/L  Description
---------        ----------                        ---  -----------
eth0             192.168.61.3/24                    u/u
lo               127.0.0.1/8                        u/u
                 ::1/128
vtun0            192.168.72.1/24                    u/u
vyos@vyos-beta:~$

Going a bit farther, Quagga is picking up the ZeroTier interface. Confirmed via vtysh

vyos-beta# show interface ztREDACTED
Interface ztREDACTED is up, line protocol detection is disabled
  index 3 metric 1 mtu 2800
  flags: <UP,BROADCAST,RUNNING,MULTICAST>
  HWaddr: da:14:c0:2b:3b:75
  inet 192.168.23.2/24 broadcast 192.168.23.255
  inet6 fe80::d814:c0ff:fe2b:3b75/64
vyos-beta#

For reference here's all I did to get ZeroTier onto a VyOS box:

1. curl https://install.zerotier.com/ | bash to install ZeroTier
2. zerotier-cli join YOURNETWORKID to join ZeroTier Network
3. Authenticate the node on your ZT controller or ZeroTier Central
4. zerotier-cli listnetworks to check status.

ZeroTier was added to the Ubiquiti EdgeRouter (which runs a vyatta fork) as a demo. Relevant to this thread:
https://blog.kruyt.org/zerotier-on-edgerouter-p2/

sorry, but their licensing model incompatible with what we do.

Any movement on this? ZeroTier integration would be fantastic.

There will be no ZeroTier in VyOS due to its license
see https://www.zerotier.com/2019/10/30/on-the-gpl-to-bsl-transition/

Approaching implementation of ZeroTier from a different angle in:
https://vyos.dev/T6455