ISPconfig 3.0.5.4p5 + varnish

I'm trying to enable varnish cache to all my apache vhost (in a CentOS 7.0.1406) but i'm having troubles changing the default listen port of apache. For vhosts port i have created /usr/local/ispconfig/server/conf-custom/vhost.conf.master with the following change:
...
<tmpl_loop name="vhosts">
<tmpl_if name='port' op='==' value='80'>
<VirtualHost {tmpl_var name='ip_address'}:81>
<tmpl_else>
<VirtualHost {tmpl_var name='ip_address'}:{tmpl_var name='port'}>
</tmpl_if>
...
and this works fine for the vhosts configuration files. The problem is that this doesn't change the apache default listen port from 80 to 81 (vhost are listening in port 81 put apache is still using port 80). Is there any way i could change this using custom templates (i couldn't find it), or i should change manually in /etc/httpd/conf/httpd.conf the listen variable to 81?