Yes, this very do-able with Cantabile 3.
First you must check the default settings in Tools>Options>Audio Ports, the input ports may have automatically been created for you there but if not you can add and alter them by using the add and edit buttons
Second you can check the Tools>Options>MIDI Ports and make sure your keyboard is in the list and checked , if it provided a description of itself to Cantabile it will be in the seetings column next to the virtual Port Name. Usually If you plug a keyboard to your system, start Cantabile up , it will see your Keyboard and make it the ‘Main Keyboard’ in the Ports list an din the main programing pages. If it’s not there then add it using the functions available.
Below is an example of a setup like you described more or less that illustrates the routing inside C3 for this
In the lower Area you can see the Output Ports section where you add vsts and vsti’s. Begin by clicking on ‘Add Object’ you’ll get a menu of the vsts and vsti’s in your vst folder you’ve selected in Tools>Options>Plugin Options. When done it be in a named slot.
Next In the Input Ports section you create the Audio and Midi routes (press Add Route to get a “Name/Source” selected from drop-down list and click on the slot next to the selection you made to set the “Preset/Destination” in the same manner). Since we want to route to the vocoder plugin we select it from the drop box of Destination for both the Audio and MIDI.
For more details you could read the manuals and see the start videos at main website