By default, Oracle VM VirtualBox's NAT engine will route TCP/IP packets through the default interface assigned by the host's TCP/IP stack. The technical reason for this is that the NAT engine uses sockets for communication. If you want to change this behavior, you can tell the NAT engine to bind to a particular IP address instead. For example, use the following command:

$ VBoxManage modifyvm VM-name \
--natbindip1 "10.45.0.2"

After this, all outgoing traffic will be sent through the interface with the IP address 10.45.0.2. Ensure that this interface is up and running before changing the NAT bind address.