Similar yes, but I actually see a slight distinction be tween the two. To me, So it goes implies an outside observer describing the situation to a second outside observer. It seems as if the first observer is more knowledgable about the situation at hand and is imparting knowledge, changing the structure of their world. Nothing to be done, on the other hand, seems to come from the mouth of an active participant; someone has tried, has given up, and is now describing her experience to a fellow compatriot. The compatriot might also be explaining her own experience; the two are interchangeable. There is a mutual agreement on the futility of any action, and yet they stil act, their actions changing nothing. Of course, this is all highly subjective and incredibly dependent on your personal worldview, but this is how I interpret it.
Hey that makes a lot of sense. I could even over-analyze it further and posit that Vonnegut's emphasis on the internal, the participator, is why his novels never descended into absurdism. I always liked See the cat? See the cradle? better anyway.