They tell you to use the grease on the outside of all powerflex bushes to prevent the bracket from pinching or bunching the bush up. It's really only supposed to allow the bracket to slide easier over the bush.
Obviously you wouldn't use the grease on the inside
If they've written that in the instructions they might be getting some claims for written off cars lol
As for vibration - nope, utter b****cks. My rack is bolted directly to the subframe as it's a DCi one and there's no extra vibration through the wheel at all - just a quicker turn and a better feel