The provided example jail configs in the jng and jib scripts do not work. Multiple interfaces need to be specified as a comma separated list or using multiple += lines in jail.conf, In the given example, the whole string is used as an interface name, which doesn't work with multiple interfaces listed.
Also added a note to the README about VIMAGE being built in by default on amd64 in FreeBSD 12, with appropriate instructions for loading the necessary netgraph modules, since they are not built in and do not autoload.