Can a kit fox (Vulpes macrotis) and a red fox (Vulpes vulpes) together produce offspring?
Would such a hybrid be fertile?
Yes, a red fox and a kit fox could produce hybrid offspring, but it would be sterile.
Sterile red fox / arctic fox (V. lagopus) hybrids have been bred for the fur trade for decades (source).
Arctic foxes are quite closely related to kit foxes, and both species are equally distantly related to red foxes, so since one can produce hybrids then the other should be able to as well. Here's a screenshot from onezoom.org showing their relation. (Arctic fox = Vulpes lagopus)
Red fox / kit fox hybrids have not been observed in the wild, and kit foxes are not widely kept in captivity except in zoos, so I am not aware of any record of such hybrids. It would likely take intentional effort on the part of a fox breeder to produce them.