thanks for the reply zEitEr and nobaloney.
i have 5 IPs on this server.i will not need any features from DA for this busy site except phpmyadmin,mysql,FTP access so i can setup nginx to listen on any unused IP:80
then for FTP access create a dummy account( for non exist domain) from DA and use its public_html document root as nginx vhost document root
Is that the best way to do this?
i just don't want to disturb DA and apache