To my knowledge - the 'recommended by vets' is a historical thing - vets would recommend Chappie in the early 70's when there wasn't much else around and they've kept the tag line since.
Only other thing - I had a customer who had been prescribed Royal Prescription diet tins by her vet, she couldn't afford them and asked if we could find something similar. She brought the tin in and the tray - no ingredients listed anywhere. I phoned Canin but was told that as I wasn't a vet, they couldn't divulge ingredients. Looked at the packaging again, thinking no labelling was illegal, thought the label may be on the tin itself so I took the label off and found the ingredients list on the inside of the label. Ingredients were meat and meat derivatives, cereals and cereal derivatives, oils, fats, minerals - exactly the same as a tin of Chappie. The cynic in me thinks that vets may recommend Chappie because it is a fifth of the price of a Canin prescription diet