Affinity rules would need to be created at the vCenter level and not VCD so there's no way to do this via VCD. There are two ways to do this but both may have issues.
You could do this via a script if the hostname of the VM's within the vApp Template are consistent when deployed. You would have to create the host group and VM group ahead of time and add the new VM's to the VM group within DRS on the cluster. The script could run periodically or you could use AMQP blocking tasks to kick off a vCO workflow to update the rule as well.
We separate VM's to hosts by only presenting datastores to certain hosts within the cluster. This works as we are using Fast Provisioning. All datastores are members of the same storage profile for the Organization but some are for Linux VM's and connected to a couple hosts and the other datastores are for Windows VM's and connected to the other hosts. This way we only have to pay MS licensing for some sockets and not all. You have to watch it though b/c if a datastore becomes full a Shadow VM for the vApp Template will be created on another datastore via the selection mechanism and it could pick a datastore connected to the wrong set of hosts.
Eric