I am really not favoring Plesk as a hosting platform but it seems that all VPS companies offer it as the primary Vhost. Anyway, here is how you can expand the memory needed to run additional plugins and other fun stuff on wordpress within a plesk vhost:

you have to create a vhost.conf in /var/www/vhost/your-domaine/conf/vhost.conf

it should be something like :

php_admin_value open_basedir none
php_admin_value safe_mode 0
php_admin_value display_errors 0
php_admin_value file_uploads 1
php_admin_value allow_url_fopen 1
php_admin_value output_buffering 1
php_admin_value memory_limit 64M
php_admin_value allow_call_time_pass_reference 1
php_admin_value log_errors 0
php_admin_value error_reporting 2039
php_admin_value max_execution_time 1000
php_admin_value register_globals 0
php_admin_value register_long_arrays 1
php_admin_value short_open_tag 1
php_admin_value upload_max_filesize 20M

plus don’t forget to write
/usr/local/psa/admin/sbin/websrvmng -u –vhost-name=$domain