Thanks. I checked and bird flu is not a big issue in your state at the moment. Marek's can still infect a vaccinated chicken, though it does prevent the tumors. I'd put the probability of this being Marek's as very low due to her age and vax status. But there is another virus similar to Marek's that can start to show symptoms at this age - lymphoid leucosis. If the symptoms continue to worsen, it would give this potential more weight.
We don't want to jump right into the worst scenario, though. And we can't diagnose chickens over the internet. But we can treat the symptoms and see if it helps. This can also help to rule out the more serious causes.
You started the vitamin E. I suggest continuing with one 400 iu capsule once a day, plus vitamin B complex. Slip the E capsule and B tablet into her beak. The E should work more quickly than the B, but if this isn't something to do with an avian virus, her symptoms should begin to improve in a few days. Full recovery may take two to four weeks.
As for the full crop, to verify a crop disorder, check her crop first thing in the morning before she has the opportunity to eat or drink. If it's full and squishy, it's a yeast infection treatable with miconazole vaginal cream twice a day for seven days. If the crop is full and it's hard and lumpy, it's impacted and coconut oil is the treatment.
https://www.backyardchickens.com/ar...w-to-know-which-one-youre-dealing-with.73607/