I'm pretty sure you cannot do this. The FortiGate is a single "web server" protected by a single certificate. A VDOM is just a separate URL on the "Web server". Is there a reason you want separate certificates for each VDOM? What issue are you facing?
You cannot do that since that admin cert configuration is same to all vdom. Instead you can use wild card certificate to the admin portal and point multiple subdomains to multiple vdom IP address, so you can access different vdom with different ip address with same wild card certificate.
For example, you can sign wild card cert wtih "*.xyz.com"
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