It seems that vyos does not support port mirroring. In some routers and switch devices, supporting port mirroring can facilitate network analysis and sampling. I found that it seems that the tee module of iptables can be used for port mirroring
https://superuser.com/questions/753294/mirror-port-via-iptables
http://ipset.netfilter.org/iptables-extensions.man.html#lbDU