Let's say you want http://www.example.com/secure/ to always be sent over SSL (I presume here that both the normal and the SSL vhost have the same content). You could do this by linking to the correct page from within your HTML pages... but there will always be some user who will sneak by it that way.

NameVirtualHost *:80
<VirtualHost *:80>
   ServerName www.example.com
   Redirect permanent / https://secure.example.com/
</VirtualHost>

<VirtualHost _default_:443>
   ServerName secure.example.com
   DocumentRoot /usr/local/apache2/htdocs
   SSLEngine On
   # etc...
</VirtualHost>