If you enable a payment method that does require the checkout on a secure server, then the second stage of checkout will automatically switch to a secure server. As during testing you may not have a secure certificate installed, we have provided a method for you to override this. In your vsadmin/includes.php file set the following...
$pathtossl = "http://www.yoursite.com";
By doing this you will force the second stage of checkout to remain on a non secure (http) connection

One thing to note is that if you are on an https connection and then click on say the link to the categories.asp page, you will remain on the https connection which is not really desirable. To overcome this you can make all your links absolute. For instance, don´t link to categories.php but rather link to http://www.yoursite.com/categories.php

Some hosting providers may provide shared space on their secure server. Generally this is a cheaper option as you don´t have to purchase your own secure certificate, but it is a bit more difficult to set up.

The first thing you must make sure of is that you can access your database from both the secure and the non-secure areas. Your host can tell you if this is possible when you contract the secure space.

You will need to make a copy to your secure area of the thanks.php and cart.php pages. Also, you will need any images from your images directory that are used by these pages and the whole vsadmin directory on the secure space.

Now set the pathtossl, for instance...
$pathtossl = "https://secure.yourhost.com/yourdir/";
$pathtossl = "https://yoursubdomain.yourhost.com/";
As above, remember to make links in your site absolute to make sure that your customers do not get stuck on the secure portion of your site.