I remember my reaction to reading Baudelaire’s prose poems for the first time: Great! No rules.
Actually, I feel that still holds true.
But what differs a prose poem from a short short story (which I like to call a sketch, or even a vignette)? In truth, I’m not exactly sure.
But I do believe that the sketch carries a deeper sense of narrative than does the prose poem, which I find to be a word-dance more than anything, swirling around the central feeling that is the message, the thing dreamed or seen.