No vibration on my truck which has about 50K on it.
As for the engine brake, it works but you have to learn the trick of getting it to kick in. First the truck has to be in tow/haul mode. Then when you start down a hill you give the brake pedal a solid hard quick tap. Hopefully that engages the EB and if if does you'll feel it. You can then take your foot off the accelerator and coast down the hill at about the same speed that you engaged the EB at. I usually aim for 45. If it doesn't engage then try some more taps on the brake. Not the best system which is why they added a switch in 2015 model.