Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
I suspect that your Lavalier microphone is defective.
What I am really thinking is, that the Lavalier's defect is predominating over the other microphone.
Either that or, you may perhaps need to change microphone settings in the camera... I'm not overly sure what the settings available for your model of Vixia are, or even if there are any. But it's worth checking on.