I found it all came down to the security settings in the admin panel.
I had to change a setting for this group:
Unregistered / Not Logged In
I had to allow this group to view the board
I had previously disabled all of their rights. Now I have enabled only the allow them to view the board.
I have been able to now embed the cal.php file inside files outside of vb and the forums directory.
This will explain why some people could get it to work and others could not.
Cheers - i hope this helps