Hi
Unless your system is different from mine, when you plug in your microphone, the speaker output is automatically cut, presumably to prevent feedback. VP (or Windows) won't therefore allow you to hear the video track AND add a narration at the same time. You could always record your narration on another recording device.....like your camera or a separate recorder. However you would need to use earphones to hear your video soundtrack from your PC or THAT would record through the mike as well. (Obviously it would be no use turning down your loudspeakers and narrating as this would be the same as using VP........)
To test...play a sound clip through your speakers and plug in your mike while it is playing......I am pretty certain you will find the sound out will be cut.
The sound file you record separately can fairly easily be added to a second sound track of your project and then mixed in with the original.
Nat