It's best to give the vitamins directly instead of in the water so you know they're getting the full dose. It's not too hard to get them to swallow pills.
So no balanced feed made specifically for chickens such as layer feed or all flock feed? Are you able to go get Poultry Cell (Tractor Supply sells it, if you are in the US) or a vitamin E & B supplements as @Eggcessive suggested?
I can see the video. It looks like something neurological, possibly wry neck. Do you have Rooster Booster Poultry Cell?
Also, what do you feed, including treats and how often?
You'll probably have to upload your video somewhere else and link it here.
Chickens are prey animals and hide their illnesses. So if a chicken is acting off it can no longer hide that it isn't feeling well and may be more sick than you think.