I've not had the vibration when it's changing down. It won't be the problem the 2009 autos had with a bodgey TCC on the torque converter - that caused vibration at around 80-90km/h and was caused by a TCC solenoid not engaging properly.
The TCC solenoid might be what is getting your attention at about 80km/h too, although it is probably working fine. The Nissan auto box does things a little differently (others might do it too now, but traditionally autos don't do things like this). Let me explain.
The auto gearbox gets input from the engine, and through the torque converter it will rotate the input shaft of the gearbox a certain percentage of the engine rotations. That's the primary reason why auto model cars were not as fuel efficient as manuals. In a manual, 100 engine revolutions equated to 100 gearbox input revs. In the auto, 100 revs of the engine equated to 60, or 70, or 80 - it depended on the oil, the tolerances in the torque converter etc etc.
Nissan are now using a locking mechanism - the TCC (Torque Converter Clutch). This fully engages the engine to the gearbox so that 100 engine revs translates 100% into the gearbox. This only happens under certain conditions - about 75km/h or more, gear selector in "D", overdrive is switched ON and throttle at less than 50%.
To see that it's working - drive along at 50km/h and gently depress and release the throttle. The engine revs should change through a range of about 1,000rpm.
Now repeat this at 80km/h on a level road. As you vary the accelerator pressure, the engine revs hardly change at all - that is the locked torque converter.
It might seem like an early change, but maybe you're being light on the foot. That's not always a bad thing!