Eggie, vaccinated chicks can not shed the virus, only if they have been exposed to it. The vaccine does not give them Marek's. The worst thing is that a vaccinated chick can grow up, be exposed and carry the virus, and never have a symptom because they were vaccinated.I should add that the doctor told me the vaccinated chickens would only 'shed' the Marek's if stressed and non-vaccinated chickens could be susceptible if they were around them. So maybe its like a person getting shingles after the chicken pox, not common, but possible under extreme stress or advanced age?
From what I've seen with my Marek's nightmare, it worked like a contagious virus, not like shingles .
However, I have heard the theory that they all carry it like shingles and stress causes shedding. But none of mine were stressed when they got it. I think this other scientist is more correct in saying you have to assume all birds have been exposed, so any of them are capable of shedding virus. I had a "closed" flock for 2 years and never had a problem. Then I went and bought a pullet. That's when I started having problems, and the worst has been the last 2 years.
But I think your chicks are too young for Marek's and it doesn't sound like coccidiosis.
If it were me, based on that vomit, I would toss all the food, and the shavings. They may have been contaminated, moldy, poisonous. Go buy new food and shavings,
And I would always give chicks access to an area in the brooder that is 85 degrees . And let them decide what they want.
Update, please!