Dinner party etiquette. Is it the host or the guests responsibility to communicate dietary restrictions of the guests?

I feel really bad. We live in a pretty red neck town but have recently made some more progressive friends. We had two couples over for a light supper of soup and salad last night and I didn’t bother to ask if anyone was vegetarian.
The soup had ham in it. They were very gracious, but I didn’t find out until dinner was over that they were vegetarian.
Who’s responsibility is it to communicate this when making new friends?