After a long trial & error session, I think I finally understood what’s wrong.
The problem lies in this instruction notificationBuilder.setDefaults(Notification.DEFAULT_ALL).
No matter what parameter you pass to notificationBuilder.setVibrate() after setting DEFAULT_ALL or DEFAULT_VIBRATE will be silently discarded. Someone at Google must have decided to give a higher precedence to setDefaults than to setVibrate.
This is how I ended up disabling vibration for notifications in my app:
notificationBuilder.setDefaults(Notification.DEFAULT_LIGHT | Notification.DEFAULT_SOUND)
.setVibrate(new long[]{0L}); // Passing null here silently fails
This works but doesn’t feel right to initialize a new long[] just to disable the vibration.