You can change the logging level in ESXi, but this affects everything, not just what gets output to your remote syslog server.
http://dewjam.wordpress.com/2012/05/17/esxi-syslog-setting/
I don't think you can filter out the noise from the syslog daemon in 5.x. I think 4.x used the old syslogd/syslog.conf, so you might be able to edit the syslog.conf accordingly on the older hosts, but I'm pretty sure that is not an option for vmsyslogd in 5.x.