I think Vonnegut is the best author of the last half of the 20th Century, domestic or foreign. Hemingway I am less fond of, but some of his stuff (The Sun Also Rises comes immediately to mind) is very good.
As for Vonnegut, the structure of his stories were completely different than anything else ever published. His early stuff was less experimental and more straightforward (Player Piano, Cats Cradle) but by the time he got to Slaughterhouse-Five, wow, talk about really, fundamentally, altering the playing field of the American novel! It WAS different. The different makes it great. I think the plot "structure" of Slaughterhouse-Five (and other Vonnegut works) is why every attempt to make them into a movie is an utter failure. It can't be captured linerally, as is necessary for a movie.
I remember one time walking through the Metropolitan Museum of Art with my mother. The Met has several Pollock's on display. My mother couldn't believe that these were considered great art. Now, admittedly, Jackson Pollock's work seems a little more blase, but in 1950, no one had ever done anything like that before. Pollock, in no small part, "created" modern art. I think Vonnegut holds a similar place in literature as does Pollock in the world of art. Hemingway and Vonnegut changed the way novels are written and looked at. They are titans.