Blanch green beans in pot of boiling salted water til tender, about 4 minutes; drain. Cook bacon in skillet over medium heat til crisp. transfer bacon paper towel lined plate and drain all but 2 Tbsp. drippings from skillet.
Add mushrooms and shallots to skillet and cook over medium heat til mushrooms are browned and shallots are softened, 8 to 10 minutes. Deglaze skillet with vinegar, scraping up any browned bits. Reduce vinegar e=til evaporated, then add reserved beans to skillet and toss to mix. top beans with bacon and season with salt and pepper.