Bless your heart! I think that sometimes we get intimidated by certain personalities and/or positions of authority. As an adult, with kids in school, I used to be intimidated into silence when I had to meet with the PRINCIPAL! It was so scary to me to be "sent to the principal". Then I realized that it was MY kid and I needed to be there in the best interests of my child, even if it meant speaking up to the mean scary principal
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So I truly understand your situation. Vets are given very little training in nutrition, and as long as you are feeding good foods that provide 100% of the nutritional requirements, and your dog is doing well on your selected diet, I say mix away. You know what works at your house as far as house training, so do what works for you.
I would hope that the vet really means well, but he may be one of the "holier than thou" types that believes he knows all, sees all.
Glad you had breakfast and coffee and are feeling better. We all have our moments.