In general, in Apache your first host is the one that will get displayed as default if no ServerName or ServerAlias.
May distros include a 000-default.conf that becomes the default vhost.
Now, for your particular needs, Looks like you're setting up a dev environment. You can use DNSMasq and apache to automagically resolve .local to localhost and tell apache to look for it under a particular directory:
wp.local -> /var/www/wp
something.local -> /var/www/something
# catch .local
Options Indexes MultiViews FollowSymLinks
Require all granted
See this post for further reference. He's using OS X but should work for any *nix distribution.