This how my vhost looks:

NameVirtualHost *:80

<VirtualHost :80>
        ServerName misitio.com
        ServerAlias www.misitio.com
        ServerAdmin miemail@yahoo.com
        DocumentRoot /var/www/misitio.com
        ErrorLog /var/log/apache2/misitio.com.error.log
        CustomLog /var/log/apache2/misitio.com.access.log combined

        <Directory />
                Options FollowSymLinks
                AllowOverride None
        </Directory>

        <Directory /var/www/misitio.com>
                Options Indexes FollowSymLinks MultiViews
                AllowOverride All
                Order allow,deny
                allow from all
        </Directory>
        LogLevel warn
        ServerSignature On
</VirtualHost>

<VirtualHost *:80>
        ServerName subdomain.misitio.com
        ServerAdmin miemail@yahoo.com
        DocumentRoot /var/www/misitio.com
        ErrorLog /var/log/apache2/subdomain.misitio.com.error.log
        CustomLog /var/log/apache2/subdomain.misitio.com.access.log combined

        <Directory />
                Options FollowSymLinks
                AllowOverride None
        </Directory>

        <Directory /var/www/subdomain.misitio.com>
                Options Indexes FollowSymLinks MultiViews
                AllowOverride All
                Order allow,deny
                allow from all
        </Directory>
        LogLevel warn
        ServerSignature On
</VirtualHost>