Input Director is essentially a software-based KVM switch, right? It has to be installed on both PC's, so they can communicate with each other and also catch the input from your keyboard+mouse. If I understand correctly: by "switching monitors", you're saying the Input Director stops signals from your keyboard+mouse from going to PC1 and instead sends them to PC2. Pressing the "M" key on your keyboard now triggers Input Director on PC2, sending control back to PC1.
My assumption/theory: Voice Attack on PC1 is still trying to "push" M on PC1, sending control to PC2. You're already on PC2, so from your perspective nothing seems to happen. (Voice Attack doesn't physically push anything on your keyboard, so PC2 is not receiving any input from Voice Attack.)
Do you have Voice Attack on PC2? If not, I suggest installing it and your control profile onto PC2 as well.
Does Input Director redirect your microphone to PC2? If it does, easy to move on from there. If not, you may need a secondary microphone added to PC2, and edit your VA profile to stop listening to whichever PC is no longer primary.