there really is not one...even though the Vineyard room does a higher scale on the food end they really arent stuffy at all and Granvilles is pretty down to earth as well...I think its totally up to you
We just ate at Granville's, Napa Rose, and Blue Bayou. I called to see if there was a dress code, and I was told none of the restaurants in DLR had one.
With that said, we did return to the room to dress for Granville's and Napa Rose. We scoped both places out ahead of time, and they looked a bit more formal.
We didn't worry about Blue Bayou, as that is in a theme park.
Although not required, you may feel out of place if you are too casual at Granville's.