Ok, I finally was forced to go read: http://simplesamlphp.org/docs/stable/simplesamlphp-install
It's not a plugin to Moodle but a separate service to itself. You say you already have localhost working. And that the URL to moodle is localhost/moodle/. To do that, you did NOT have to set up virtual host. Moodle folder is being served out just fine. This other app recommends you run virtual apache ... virtual apache will work ONLY WITH DNS that functions ... localhost to ANY machine connected to the internet means IP is 127.0.0.1 (the loopback IP of ALL machines of typical configuration on the Internet). The only other way to get localhost to respond with Virtual apaches by another host is to edit your hosts file and manually enter a 127.0.0.1 IP mapped to a simplesami.localhost server as well.
So suggest to you, that installing this as a subdirectory at the same level as the moodle folder is perhaps the easiest way to go. According to their install docs: You may choose another location, but this requires a path update in a few files. See Appendix for details ‹Installing simpleSAMLphp in alternative locations›.
You then don't have to attempt to configure virtual hosting for localhost.
BTW, the docs to that thing show linux and you are using Windows? Wouldn't it be best to set it up on Linux for testing? You could run Ubuntu/CentOS ... or almost any Linux distro ... in a VirtualBox virtual environment and then you'd be working in the VERY SAME environment as your production server. Think that would be a more accurate testing environment - especially for something like this.
Since this really isn't Moodle related, does the provider of this thing have a forum of their own? Or contact information? If so, suggest your best advise will come from there/them.
my 2 cents.
'spirit of sharing', Ken