/parallels_plesk_panel_installation_directory/admin/sbin/websrvmng --reconfigure-vhost --vhost-name=
On Centos 5
/usr/local/psa/admin/sbin/websrvmng -u --vhost-name=domain.com
In Plesk 10
/usr/local/psa/admin/sbin/httpdmng --reconfigure-domain domain.com