@casportpony
The not eating or drinking thing worries me...and tubing a little one would need some very good information so you don't harm the baby . . . casportpony has experience with tube feeding.
Do you have a scale that would enable you to get a good weight reading? Tracking weight may really matter.
What kind of vitamins? Sounds like this could be B vitamin deficiency-related, but there are other things that can happen. I would start B vitamin supplements right away. Down below I'll copy something I wrote up about B vitamin supplements.
If you can get the little one to a vet a.s.a.p., I would do that. It could be an infection, a congenital health issue, conceivably the little may have eaten something that got stuck in the digestive tract . . . . are there any odd bumps? how does the vent look?
Supplement writeup
Ducklings need about three times the niacin chicks do. And some ducklings get leg problems or seizures if they don't have enough niacin.
You can use brewer's yeast. Forgive me for using capital letters, but for anyone else who may casually look over this thread, NOT baking yeast, NOT winemaking yeast. Brewer's yeast is a nutritional supplement and it provides niacin and a few other vitamins.
Generally, folks who supplement with brewer’s yeast use a tablespoon per cup of food.
If you cannot find the brewer's yeast, or if you happen to have (here come the capital letters again, please forgive me) PLAIN niacin - NOT no-flush, NOT timed release, then dissolve 150 mg niacin per gallon of water for the duckling’s drinking water.
Another approach is to use B Complex capsules, and use the B3 - niacin - levels as a guide. Again, 150 mg B3 per gallon of drinking water.
Keep the little one on the supplements for about 8 weeks.
Sometimes it only takes a few days to see improvement.
You can also see how it went for Qwackers and Bentley from these threads.
https://www.backyardchickens.com/t/857143/treating-wry-neck-in-crested-duckling-edited-title
https://www.backyardchickens.com/t/761308/peking-duckling-with-crooked-neck-help