The message you need to monitor is actually *Configured from vty*.
I only have the logging server xxx.xxx.xxx.xxx command in my config. What output do you get when you do a show log? I have the following. I don't use a loopback address, I'm using mgmt0 interface. You may just need to set your severity level.
Logging console: enabled (Severity: critical)
Logging monitor: enabled (Severity: notifications)
Logging linecard: enabled (Severity: notifications)
Logging fex: enabled (Severity: notifications)
Logging timestamp: Seconds
Logging loopback : disabled
Logging server: enabled
{xxx.xxx.xxx.xxx}
server severity: notifications
server facility: local7
server VRF: management
Logging logfile: enabled
Also do a show log to see if you can see the syslog "configured from vty" after you've been in conf t mode. If it's there, then it should also be sent to the syslog server.