It appears to be the one in Massachusetts. The essay is frequently cited as being published in Wheaton College alumnae magazine, which is the magazine of the Wheaton College in Massachusetts.
I note, however, that the library's own history page does not make any mention of a rededication in 1973. There was a renaming of a room after a key alumna in 1972.
It's admittedly been a little while since I read the book, but here are a few thoughts.
First, Billy Pilgrim is coming unglued at this point. Keep in mind that Billy Pilgrim survived a major plane crash, which appears to have caused some degree of brain damage. That being said, we obviously can't take Billy Pilgrim's view of events entirely seriously.