my dog has mvd as well with no symptoms she is on purina ha low protein hydrolized soy diet. It is the ALT that was high for your dog not ALP. You need to know the pre and post bile acid numbers. Sounds like your vet is very sharp btw. I am impressed as i know alot about this as on liver shunt/mvd group. Your vet is correct over 100 is usually a shunt and your dog would have some symptoms either bladder stones, head pressing, seizures, picky eater, spaced out after eating etc.
My dog has pre 3 and her three posts were 73,54 now 26 on low protein diet. 25 and below is normal. 21% is not bad if your dog is not having symptoms it is 22% or below and if having symptoms 18% and below so your vet is right no need to adjust diet but watch the protein count. You may want to do natural balance fish and sweet potato as fish is a more digestible protein and liver compromised dogs need to be on a highly digestible protein. But honestly if no symptoms, etc i would just keep doing what you are doing. They just need to be careful when doing anesthesia or giving meds. However my dog is on temarilp a steroid and her liver is doing fine and she is on soloxine for hypothyroid so I really would not stress too much about as your vet isn't concerned and neither am I
My dog is now 5 1/2 and still no issues.
if you want to keep the liver cleansed you can juice zucchini and celery together put pulp on food and juice and that is a natural cleans of liver by using zucchini and celery cleanses the urinary tract. I would not use milk thistle or sam-e as i do not supplement with these either since she has no symptoms and her blood work is fine however my girl did not have a high ALT just high post bile acid tests