Your chick is "preening" after you apply witch hazel to her vent because it stings like crazy. She's trying to make it stop hurting by pecking at it, much as you would do by holding a sore thumb you just got through smashing with a hammer.
Try coconut oil. It soothes, heals inflamed tissue, and will prevent infection. If the swelling refuses to go down, try Preparation H.
More to the point, her vent is swollen for a reason. Perhaps she's constipated and is straining to poop. If she is chirping madly when she squats, she's probably constipated. Pasty butt often results in constipation, too.
As a precaution, I recommend treating her with coconut oil to be sure things are moving as they should. Chill the coconut oil and then break off tiny bits and place them onto her tongue. She will automatically swallow. Get one teaspoon of the oil down her if possible.
Make sure the brooder isn't too hot and the chicks have enough room to find cool space away from the heat source. Also, be sure they are all able to access their water okay.