All of your pppd messages are getting dumped into /var/log/messages, and making a big mess, and you would rather have them going to a separate file.
Create your logfile:
# touch /var/log/ppp
Then, add the logfile option to /etc/ppp/options:
logfile /var/log/ppp
Delete any references to logfd, as the two options are mutually exclusive.
There is no downside to having separate logfiles for your services; it makes it a lot easier to find out what is happening on your system.
Customizing the standard Linux syslog is bit more difficult than it needs to be; see Chapter 19 to learn how to build a robust, easily customizable logging server with syslog-ng.
See Also
man 8 pppd