I figured that too. He isn't dead by any means but yes he itches constantly. Once he started puking and we didn't have ground beef and just fed him rice for a couple of days...he did fine. I really really REALLY am not into the vegan dog thing at all. I mean...now its certain...no argument and in textbook that dogs are carnivores lol.
The only thing I could even think of is for whatever reason he has a sensitivity to the packaging that the meat is touching? Think thats a possibility?
When you did raw, did you always feed a bunch of meats? It might be worth picking one (something novel, like fresh sardines, rabbit, duck, etc.) and sticking with it for several weeks to see what happens. What meats did you used to feed? Chicken tends to be the biggest cause of allergies (honestly, it's probably more the additives than the chicken for a lot of dogs). Where did you get the meat and did you check the sodium content? Some meat has a saline solution added, this will cause itchiness and/or puking in many dogs. Your mention of ground beef makes me wonder if it's meat from a grocery store that does that. Some brands do, some don't.
Has the vet tried prescribing any allergy medications? It didn't sound (from your post) that environmental allergies have been ruled out yet.
I've never heard of something like that but anything's possible. My hunch is that it's environmental or otherwise totally unrelated to the food OR it's a meat but not meat in general (so something that was part of his diet but he didn't get it every meal). He wouldn't have done fine sometimes on a meat-based diet if it was a true allergic reaction. I have a dog who is allergic to wheat...a single piece of bread is enough to make her sick for days. It's never a random thing, she always gets the same reaction whenever she has something with wheat. Someone I know on another forum has a dog with a chicken allergy. ANY time he gets chicken, he reacts (rashes, constant itching, teary eyes), no matter the source. If your pup isn't reacting every time, something odd is going on.
It might help to start recording everything he eats (including treats), when he has a reaction, and what he did that day. A pattern is bound to emerge eventually. My sister's dog gets allergic reactions whenever there's a lot of pollen, for example. I fostered a dog who had allergic reactions after running around outside (from the grass). My senior dog who recently passed away couldn't tolerate turkey at all...no idea why but it turned out that every time she got sick it was the day after eating turkey.