<VirtualHost 1.2.3.4:80>
RewriteEngine On
Redirect 301 / http://www.mainsite.com/
DocumentRoot "/var/www/html"
ServerName site1.com
ServerAlias www.site1.com
<Directory "/var/www/html">
allow from all
Options +Indexes
</Directory>
</VirtualHost>

<VirtualHost 1.2.3.4:80>
RewriteEngine On
Redirect 301 / http://www.mainsite.com/
DocumentRoot "/var/www/html"
ServerName site2.net
ServerAlias www.site2.net
<Directory "/var/www/html">
allow from all
Options +Indexes
</Directory>
</VirtualHost>

RewriteCond %{HTTP_HOST} .
RewriteCond %{HTTP_HOST} !^www\.
RewriteRule ^/(.*)$ http://www.%{HTTP_HOST}/$1 [R=301,L]