Nana, I don't think there's any extra charge for the aft cabins -- a "C" is a "C" for pricing. It certainly was for us, and I haven't seen anything indicating they've created a different cabin class for these few cabins.
Going back to my earlier posts, I need to correct two things I said above. First, there *is* a full-length mirror! It's in the area between the hallway closets and the entertainment center wall unit. When I responded to Pat's comment, I was downstairs on Deck 5, and just assumed there wasn't a mirror -- but I slapped my head when I returned to the cabin later and saw it was most certainly there.
Also, there *is* a little bit of soot on the balcony. So little that I didn't notice until after our stewardess pointed it out while cleaning one day. It's not big flakes of soot, just tiny little dots, not much of it, and only at the very rear of the balcony. To me, this was a non-issue -- no more a problem than sitting on deck chairs at the (uncovered) outside of La Palette lounge on Deck 8.
And now that the cruise is over, I'll reiterate that there were no problems with vibrations at any time as far as we noticed. In fact, I expected to at least feel some rumble from the engines, as we did just a little on the late Radisson Diamond, but I found it very quiet. When the ship is at full speed, you can hear some noise of water passing the hull from the main dining room at the back of Deck 5, yet you can't hear any such noise in the cabin on Deck 7.
-- Eric