I would not worry about the Purina Pro Plan....I don't think it is necessarily a "bad food". My pups eat different foods that are given bad ratings by the folks who think they are experts. I don't and won't make excuses for my choices. I don't think there are a lot of "bad foods" on the market...it comes down to what works for your pup. For me, quality control is number one...and ingredients can be looked at after that. That guy who runs the one food site is a human dentist. He focuses on ingredients without any points for feeding trials and quality control. I ignore him and his ratings. I have seen many 5 star foods on recall lists. There is another "holistic/natural" article being passed around that says prescription dog foods are bad...this is pure unadulterated hogwash and I hope they get sued. My Olivia has renal disease and has been on Purina NF for years and it is on that idiotic list.......in 2013 she was given one year to live and that is when she started on that food...she is still here.
As to vitamins, I personally don't give extra vitamins. A good dog food should be sufficient unless your pup has had a condition that a vet recommends be treated with a supplement. I don't think any harm would come .. but, it is true that some vitamins can be overdone and cause medical problems. A daily vitamin should not do that. Here is information about Pro Plan that explains how the food meets nutritional standards.
https://www.proplan.com/dogs/dog-car...od-regulations