If your users will have no direct access to the server this step should not be necessary. Should you choose to go forward, this guide seems to have all the bases covered on how to configure PHP-FPM for this type of use-case.

I am going to work on this same scenario on a debian 8 install. I intend to follow this - https://www.digitalocean.com/community/tutorials/how-to-set-up-nginx-server-blocks-virtual-hosts-on-ubuntu-16-04

Thanks ryanpq for the quick reply.
I understood your point of view regarding security, but I would like to know how to isolate the websites in the new environment that is Nginx with HHVM, maybe I was not clear enough in my initial question. Thank you anyway!

Hi, I finally go it going.
You need one hhvm installation and the rest will work. Follow these tuts:

https://www.howtoforge.com/tutorial/install-wordpress-nginx-mariadb-hhvm-debian-8/
https://www.digitalocean.com/community/tutorials/how-to-set-up-wordpress-multisite-with-nginx-on-ubuntu-14-04

I strongly recommend you use MariaDB instead of MySQL

And the Nginx will look like these FOR EACH SITE. Place the blocks at /etc/nginx/sites-available/default (you can make a copy of default and then delete everything)

#Start of server blocks FOR EACH DOMAIN

# HTTP - redirect all requests to HTTPS:
server {
        listen 80;
        #listen [::]:80 default_server ipv6only=on;
        server_name another.com;
        return 301 https://$server_name$request_uri;
}

# HTTPS - proxy requests on to wp app:
server {
        listen 443;
        server_name another.com;

        ssl on;
        # Use certificate and key provided by Let's Encrypt:
        ssl_certificate /etc/letsencrypt/live/another.com/fullchain.pem;
        ssl_certificate_key /etc/letsencrypt/live/another.com/privkey.pem;
        ssl_session_timeout 5m;
        ssl_protocols TLSv1 TLSv1.1 TLSv1.2;
        ssl_prefer_server_ciphers on;
        ssl_ciphers 'EECDH+AESGCM:EDH+AESGCM:AES256+EECDH:AES256+EDH';

        root /var/www/html/wordpress;
        include hhvm.conf;

        # Add index.php to the list if you are using PHP
        index index.php index.html index.htm index.nginx-debian.html;

        location / {
                try_files $uri $uri/ =404;
        }

        location ~ /favicon.ico {
            access_log off;
            log_not_found off;
        }

        # pass the PHP scripts to FastCGI server listening on sockets

        location ~ \.php$ {
                include snippets/fastcgi-php.conf;
                # With php7.0-fpm:
                fastcgi_pass unix:/var/run/php/php7.0-fpm.sock;
        }

        access_log  /var/log/nginx/$host-access.log;
        error_log   /var/log/nginx/wordpress-error.log;

}

All the best.