Hi,
Try this configuration in 2811 for syslog with logging level and facility level and check out the message are coming in syslog or not for VPN session.
Router# configure terminal
Router(config)#logging host
Router(config)# logging trap level
Specifies the kind of messages, by severity level, to be sent to the syslog server. The default is informational and lower. The possible values for level are as follows:
Emergency: 0
Alert: 1
Critical: 2
Error: 3
Warning: 4
Notice: 5
Informational: 6
Debug: 7
Use the debug level with caution, because it can generate a large amount of syslog traffic in a busy network.
Router(config)# logging facility facility-type
Specifies the facility level used by the syslog messages; the default is local7. Possible values are local0, local1, local2, local3, local4, local5, local6, and local7.
Router# show logging
Displays logging configuration
and check out the below link for facilty codes for syslog messages hope this helps out your query !!
http://www.monitorware.com/common/en/articles/syslog-described.php
Regards
Ganesh.H