The European Vegetarian Union has put forward two labels, which have already been taken up by many continental European brands. From my personal experience, these labels seem to be the most common ones at in Germany and Austria at the moment. The "vegan sunflower" mentioned in another answer is a close second.
One downside of the label is that it uses the same symbol for vegan and vegetarian products, separating the two only by the word "VEGAN" or "VEGETARIAN", respectively.
