I should have done this before. I like doing a Google site search of Project Gutenberg, which searches a big corpus of books published before 1923, to check for examples of usage. Here are some non-Vonnegut examples, and some use the U.K. variant Michael mentioned, "so it goes on." I wasn't quite right; it doesn't always express resignation, sometimes it's just way of saying "that's the everlasting routine" or "that's the way it is in the world."
site:www.gutenberg.org "so it goes"
"So it goes, and so it went, from year to year: a little showing now and then, like the iceberg's tip, from which to guess the bulk below."
"[A dishonest grocer would go to jail]. And so it goes in every business but finance--finance, the most important of all..."
(About obesity) "...riding a horse doesn't reduce a fat man. It merely reduces the horse.
So it goes—the fat man is always up against it. His figure is half-masted in regretful memory of the proportions he had once..."
Here's a good one; Robert Frost (U.S. poet and Nobel laureate), free verse. I think this is very close to the way Kurt Vonnegut uses the phrase.
"For, dear me, why abandon a belief
Merely because it ceases to be true.
Cling to it long enough, and not a doubt
It will turn true again, for so it goes.
Most of the change we think we see in life
Is due to truths being in and out of favour."