Hi Vishal, thanks for your advice. It didn't solve my problem per se, but at least taught me something new AND confirmed that there must be something else that I'm doing wrong. So far the new progress I've made today is:

I know now that I can use .html files to process ssi after creating a test file and seeing how it works. I also got curious as to how an html file can process a php file inside an ssi code, so made a test.html file that has the following code: "<!--#include virtual="/vb.php"-->"

When I went online to look at the result, it says: "[an error occurred while processing this directive]"

Well whatever that means, it means the ssi works with .html files, yay! So anyway, since I now know that the add.cgi file can use the login.html template to process the "<!--#include virtual="/vb.php"-->" part, I tested it out but when I view the source of the actual login page, the "<!--#include virtual="/vb.php"-->" part STILL shows as is.

So what the hell am I missing here? Please help, thank you.