Enable Vibrate, then Ring
The stock Nexus only has Vibrate or Vibrate & Ring. To enable Vibrate then ring, edit your desired ringtone to pad a few seconds of silence at the beginning. Then set phone sound to Vibrate & Ring.
Here's a link
to one I made, 4 seconds of silence, then fade in a digital ringer. I used Audacity
to edit the ringtone.