I just wanted to post about recent experience with Vacantionstogo.com, and thank Elliot for recommending them. It turns out my mother also recommended them, but I hadn't paid attention, ha!
But yes, just booked with them for the NCL Epic transatlantic and the people are so nice at VtoGo, it really impressed me. I had to cll several times for various reasons, and they always answered quickly and in very good moods. All Americans, too! Clearly they are taking care if their employees (I think it's the only way to get such a good attitude out if employees) and they got me a better price after calling than I saw anywhere else on the Internet.
Will be using them again!