Can I secure SSL VPN with a wildcard cert, if SSL VPN is IP based?

Yes you can, but you need to access it via FQDN (e.g.: ssl.mydomain.com), not IP address, otherwise the cert is useless.

The IP address will not match the certificate wildcard subject name or alternative subject name. The wildcard certificate cannot be used to authenticate the server by its IP address.

the wildcard certificate will only apply if the method you are connecting to the sslvpn is fqdn based. in this case, you'll have to bind your public ip with fqdn or use FortiDDNS: https://docs.fortinet.com/document/fortigate/7.6.2/administration-guide/685361/ddns 

Hi,

You can do this if you add the IP address in the SAN name field.
However it's better to use a FQDN.