Shared Drupal 7 core and shared contrib libraries & modules
You can symlink the same core into several drupal installs. This works cross vhost.
You can also run several sites on the same Drupal Core in a multi-site fashion. This however requires you to keep a single vhost, so you need to restructure the server config for this.


Note that I recommend against clustering to many sites. If you break a single core, you break all of them. So you would do well to run, say 5, sites per core. Obviously the number should be adjusted depending on how much resources they require and how many sites you run in total.