You can't do this with a standard VIP but will be able to do it using virtual servers/load balancer, which are a special type of VIP.
You need to enable 'Load Balance' feature in the GUI first via System > Feature Visibility > Load Balance:
Once enabled you'll be able to configure virtual servers, with a single VIP. Select HTTP Host as the load balancing method, then add your real backend servers with their hostnames.
You'll need to upload a wildcard certificate for *.mydomain.com to match both hosts.