Can you just confirm so I can be sure I understand you, you want to be able to reach the FortiGate on HTTPS (Web VPN, admin GUI, etc) using a hostname that points to its public IP address.
And you want to create a VIP with a web server behind it that is using another hostname but using the same IP address as the FortiGate WAN?
Yes this is possible! However, you need to understand that you'll need to use different ports if you want to share the same IP and port for two different servers.
Your FortiGate and Web Server are two different servers sharing the same IP address.
If you can't use a non-standard port for either the FortiGate or the Web Server, you could possibly look to using the Web Server to do some redirection for you based on the hostname. I.e. the web server could handle all port 443 connections and anything for Fortigate.example.com would get sent back to the FortiGate.