Without being able to examine the hen in person so to speak all one can do is mention possible causes and the standard treatments for them.
So, if you are really worried and she starts looking and acting sick the best course of action is to take her to a vet.
From your description of her behavior and what little I can see from the pictures it doesn't look like a problem with her vent. To me, the reddened skin below the vent looks like an irritation. Some of the older hens here get something that looks similar, particularly during the summer months. As far as I've been able to tell this rash is caused by poop sticking to the feathers, drying out and pulling on the feathers where the poop forms a solid lump. I would tend to agree with
@dawg53 and guess that uric acid could be responsible for producing the rash.
Here the older hens that have problems getting the poop to clear the feathers below their vent when they poop because they have fat rear ends.
It doesn't seem to stop them enjoying their food, laying eggs or otherwise interfere in their day to day life. It does interfere with mine because I have to clean there rear ends on a fairly regular basis and the poop sets as hard as concrete.
I would keep the area clean using water only. There are various salves and ointments but many of these, particularly those containing steroids can cause irritation.
I would put her back with the flock. As a rule I try not to separate either sick or injured chickens from their flock unless absolutely necessary. It causes stress.
If she shows signs of being ill, not eating, lethargic, separating, herself from the others, then that would be the time to investigate other possible causes.
