Get Your Dog Care Questions Answered by Experts
I really think that you need to give the breeder a call back and see if she can tell you anything about the dog's condition.
She would not have gotten a yeast infection from the vaccine but she could have some vaginitis or another problem that you are unaware of.
At this age, the dog should still be with her mother. They should stay with Mom until 8 weeks of age.
Boxers are prone to skin allergies so you will want to see your vet to rule this out and to discuss your treatment options.
You can bathe her using a gentle puppy shampoo.
Let me know if you have more questions.
I am worried that you have gotten a dog that will be trouble for a lifetime. If dogs have skin problems this early in life, they will normally have them for the rest of their life.
You can mix some replacement milk in with his food and you should not need to bottle feed. Just be sure to feed him several times a day.
Let me know if you still have questions.