I am very sorry to hear that Fox is experiencing these problems, I am sure it is very unsettling for you both. There are a number of potential causes of such clinical signs, including dietary indiscretion, pancreatitis, parasite infestation, infection, and ingestion of a toxin or poison.
I understand tha medical costs can be overwhelming, and here is a link to the Humane Society, They have resources available for low costs veterinary care;
http://www.humanesociety.org/animals/resources/tips/trouble_affording_pet.htmlIn the meantime at home try the following;
Worming treatment from a pet store, use a broad spectrum as you will not know what worms Fox has potentially, without a trip to the veterinarian first.
Rest Fox's gastrointestinal tract by holding back food for a few hours. Then you can continue with a bland and easily digested food of the boiled chicken breast and plenty of fresh water.
If Fox does not improve, has profuse vomiting or diarrhoea, you notice fresh blood, Foxseems lethargic, in pain or depressed I would advise a trip to the veterinarian.
Here are some links to helpful resources for you;
https://www.petcoach.co/article/most-common-causes-of-diarrhea-in-cats/https://www.petcoach.co/article/6-things-you-can-do-at-home-if-your-pet-has-diarrhea/I hope that answer was helpful and more importantly that Fox feels better soon. Warmest regards to you both.