It is not possible to exclude a particular virtual machine network adapter (vNIC) from the DFW. The exclusion list will disable the DFW on the virtual machine. If a virtual machine has multiple vNICs and is added to the exclusion list, all the vNICs will be excluded from DFW.
The current behavior of the DFW exclusion list will not exclude a new adapter that is added after the VM is added to the exclusion list. To exclude the newly-added network adapter, the virtual machine must be removed from the exclusion list and added back. The other alternative is to power off the virtual machine and power it on.