The yawning is problem them opening up their ear passages. A lot of people panic and think that the birds have gapeworm when they do that. They'll thrust their head forward, mouth wide open, and at the end of the thurst they really open their mouth the most - yes kinda like a severe dry heave. That's how they open up their ear canal if they're itchy. Again, I'd check for mites. Especially if they shake their head much. That being said, they're also getting pinfeathers in. They'll get them in their ear openings and they itch. My babies are also losing a feather here or there. That's normal. Losing feathers to the point of thinning would not be.
On the vitamins, I really don't like vitaminized water at all. I only use it with a distress birds, and then mostly for the electrolytes or to cover my bases in case my food wasn't fresh and something degraded. Added vitamins are usually mostlly unnecessary in a healthy flock if the food is fresh, stored correctly, and the correct formulation for their age. Sometimes if birds are stressed you can give more like the a/d/e/ combination. But otherwise vitamin packages often contain sugar and can help grow bacteria in their water.
If I ever give vitamin boosters to healthy non-stressed birds, it's usually in the form of a wet-mash treat combined with probiotics so that I know they all got it, one didn't get a ton and overdose, and that it was quick so that the vitamins didn't degrade.
Mini rant: the vitamins that birds need most and that most quickly degrade in their food, A, D, E, are the ones that are oil based and they degrade even more quickly in water in the light of a waterer.
Go figure. Rant over.
As for the yogurt, I wouldn't even bother giving it tonight. But I'm glad they're back on the medicated starter. It really does help the first couple of months. After that, their gut bacteria are established and they're in bigger areas with less exposure to the wet conditions and poop exposure that encourage coccidiosis.
By the way, don't you love the little flying bit? I call that Chicken Airplane.
They try to take off, bless them. They must be feeling pretty good!