Q: to solve loopback problem
A: creating a lo1-device and then assigning each jail a loopback address of the form 127.0.1.$jailid on lo1

# /etc/rc.conf
cloned_interfaces="lo1"

# jail.conf
foo {
 jid = 1;
 ip4.addr =  "lo1|127.0.1.${jid}/32";
 ip4.addr += "vtnet0|10.0.0.${jid}/32";
}

lo0: flags=8049<UP,LOOPBACK,RUNNING,MULTICAST> metric 0 mtu 16384
# by service netif start lo1
lo1: flags=8008<LOOPBACK,MULTICAST> metric 0 mtu 16384