I have seen a fox get into a pen of chickens (my neighbors chickens) with a few golden retrievers. I’m sure it‘s not to common though, i just think if they are desperate enough they might get in with a pen of dogs. I know one with mange would because i’ve had a few mange ones by our house and they would literally do anything for food. (Though this is a different scenario because this fox doesn’t have mange, just thought it was worth mentioning.)That's one brave hawk! A fox is a completely different animal with a different risk-benefit equation though, you can't equate the two scenarios. If the hawk fails or things turn against it, it can just fly away. The fox is a ground animal and, attacking another ground animal of comparable or larger size, especially another carnivore with teeth and claws, is a huge risk with no clear path to escape. The dog can attack in self defense and give chase, and seriously injure the fox. Foxes aren't stupid, they know that.
I’m not sure where the conversation started before i had replied, i realise now your probably talking about a dog twice the size of a fox and the fox attempting to eat it being impossible, if that’s the case sorry I misunderstood. I assume you meant some self defense or a smaller dog.