"So it goes" can't be called a Vonnegut reference unless it's surrounded by other Vonnegut references, because it's a perfectly ordinary phrase in common use outside Vonnegut.

Think of it this way: "So long, and thanks for all the fish" is a Douglas Adams reference, but mentioning a dolphin isn't.

An "allusion" is an indirect reference/pun. The degree of indirection is clearly subjective. The phrasing great grandparent recognized from Adams (at least one other person) is actually a negation of the famous line from Restaurant At The End of the universe. (EDIT: which, for the record, is "The story so far: In the beginning the Universe was created. This has made a lot of people very angry and been widely regarded as a bad move.")

I do agree "So it goes" is more of a stretch here especially since dpk seems to live in Germany and it's a common translation of "So geht's" which may be more common in Germany than "So it goes" in primarily English speaking countries. I.e., how "perfectly ordinary" may be reader-dependent. I think it's a stretch supported by the Adams adaptation two paragraphs earlier, though. In primarily English speaking countries it is "signature enough" that they use it for the Vonnegut Library: https://www.vonnegutlibrary.org/so-it-goes/ , but sure it comes up a lot more than "thanks for all the fish". I think these are disputes of degree more than kind.

TL;DR - I stand by my guess, but you'd have to ask dpk to be sure.

EDIT: and Vonnegut's parents were German - so, this may also just be V being an "ambassador" making the more "perfectly ordinary" in one domain more "signature" in another domain. It happens.