VDDK will attempt to determine which transport method works for the VM in question and then use that transport method to perform the backup. In your case based on the selection you put in the policy NBD will more than likely get chosen.
The only way I can see you pulling off what you are asking for is to create two separate policy. One for OS/C drive (Exclude Data Disk), and one for Data Drive (Exclude Boot Disk). The exclude options are configurable in the policy advanced options. In the respective policies you select the transport type you would prefer to use.