Well technically the series could take place in every comic universe, since The Endless are, well, endless. Aspects of them exist everywhere.
But to answer the actual question you're asking, the original series and its spin-offs mostly took place in the mainstream DC universe (as did most of the old Vertigo titles), and it seems most likely that this new prequel series continues to take place pre-"New 52". Even though Vertigo has been rebranded as DC's "creator owned" imprint, all Sandman stuff remains with them because 1) Gaiman seems to at least partially own the characters, 2) taking The Sandman out of Vertigo would have been a marketing disaster. Doubly so if they tried to include it in the "New 52".