With the newer propulsion units most ships don't get alot of vibration even on the lower decks. I am certain that unless you are motion sensitive you probably won't even feel the ship moving, except when making hard turns, brakeing, and docking. On my last RCI cruise i only felt the ship move twice, one was a hard turn , and the other was docking in Cozumel... seems as if that channel of water gets quite alot of wind which pushes the ships a bit making docking a little more difficult..
You should be fine, your balcony is high enough that you probably won't feel much except perhaps when they reverse the azipods to slow the ship during docking. I'll be sailing the Legend in the second week of may, so i'll have a better idea then...