For an existing VSTi, click in the Inspector on the channel settings (the 'e' symbol) and at the right side of the following window you see the Cubase mixer channel in use and also the Cubase Output Channel in use. These are also written right below the title of this window. Let's say this says 'Stereo Out 2'. Now you open Cubase Audio Connections > Outputs Tab and make sure this Stereo 2 output is assigned to the SLOT1 Tascam IFFW channels of your choice.