According to the Apache's name-based virtual host documentation the first virtual host is the default host. I use this to make the first entry a catch-all that redirects every "undefined" request to the main site:
# Default catch-all
<VirtualHost *:80>
# Note the lack of a ServerName
RewriteRule ^(.*)$ http://www.example.com$1 [redirect=permanent]
</VirtualHost>
# Site 1 - www.example.com
<VirtualHost *:80>
ServerName www.example.com
[ the rest of the site config ]
</VirtualHost>
For extra credit you can set up a wildcard DNS entry so that every undefined host (e.g. asdfasdfasdfas.example.com) gets redirected to www.example.com.