Ah achou!
1) crie o vhost.conf para o subdomínio: /var/www/vhosts/xxxx.com/subdomains/test/conf/vhost.conf
2) adicione o seguinte:
<Directory /var/www/vhosts/xxxx.com/subdomains/test/httpdocs>
<IfModule sapi_apache2.c>
php_admin_value open_basedir "/var/www/vhosts/xxxx.com/subdomains/test/httpdocs:/tmp:/usr/share/pear"
</IfModule>
<IfModule mod_php5.c>
php_admin_value open_basedir "/var/www/vhosts/xxxx.com/subdomains/test/httpdocs:/tmp:/usr/share/pear"
</IfModule>
</Directory>
3) Configure o plesk para reconhecer o novo vhost.conf
/usr/local/psa/admin/sbin/httpdmng --reconfigure-domain test.xxxx.com
4) Reinicie o apache
service httpd restart