Some vibration is always normal. It's usually 'road' vibration. Check the passenger headrest the next time you're out on the highway and see if there appears to be a lot of vibration. Sometimes it can be caused by an out of balance wheel(s) but that you would normally notice in shaking of the steering wheel or in the body itself. Seeing as you've likely put on a few miles already and only noticed it after the long trip is a little puzzling.
Peter