Among all interspecies relationships, this type of relationship is the most common. Most male foxes have an intense desire to mate with a female mouse. Female mice need the male foxes to survive (even though there are male mice on Vulpes, the females outnumber the males 3 to 1.)
When a male fox and female mouse mate, It is not uncommon for the fox to nibble on the female's tail. But the male fox is very gentle with her, he will not bite her too hard. And even if he accidentally draws blood, he will not revert back to the ways of his fox ancestors. Being that the fox is part of the canine family, his genitalia is very similar to that of a dog, wolf, etc. Which can potentially cause some pain to the mouse the first time, but as they get used to each other the pain goes away. The mouse body is designed to take in high-density semen which is normally provided by the male mice. However, male foxes have evolved to produce several different types of semen including high-density semen for mice and other rodent species.
Once the female mouse has been impregnated by her fox lover, she will only be pregnant for about 1 month. Any growing fox embryos will accelerate their growth to match the development of the mouse offspring*.