This post is about creating multiple domains/sub-domains or virtual hosts on a single VPS running on Ubuntu 14.04 LTS with LAMP stack. (for LEMP stack, see another good example here.)
Summary
1) Create an A record for your sub-domain that points to your droplet
2) Create a .config file for your sub-domain site in /etc/apache2/sites-available
3) Create the directory for your site in /var/www (or wherever you defined it was in the config file)
4) Enable the site in apache with: sudo a2ensite sub.yourdomain.com (or whatever you named your site in the config)
5) reload apache: sudo service apache2 reload
NOTE: the DNS record you create might take awhile to propagate
