EricK

My wife and I are both in the middle of the N400 process (biometrics complete, waiting for interview). We have a trip to Vietnam planned for February, could this cause any issues with the application?

sun_burn

I'm currently in exactly the same status (biometrics done, waiting for interview). It won't harm your application, since you can update the record of your N400 foreign trips list at the interview. But you may risk not receiving the interview notice in time, or missing the interview somehow.