Squid access logging only specific ACLs

In addition to what otheres have suggested, Squid can be configured to talk to other proxy servers either as parents or peers. You can use a filtering proxy alongside squid to block the sites that you want to block. You can search for examples of squid+privoxy or squid+dansguardian.

Try to put the SecAuditLog entry into the setting for each of your virtual hosts.

Something like this (setup can look a little bit different but this is a basic setting). Also you could check your other log entries (CustomLog, ErrorLog etc) for possible wildcards to insert.

domain1.conf
============
<VirtualHost *:80>
  ServerAdmin postmaster@domain1.com
  ServerName www.domain1.com

  SecAuditLog "/var/www/vhosts/domain1/statistics/logs/modsec_audit.log"
  # lots of more setting goes here
  # ...
</VirtualHost>

domain2.conf
============
<VirtualHost *:80>
  ServerAdmin postmaster@domain2.com
  ServerName www.domain2.com

  SecAuditLog "/var/www/vhosts/domain2/statistics/logs/modsec_audit.log"
  # lots of more setting goes here
  # ...
</VirtualHost>