Vent Gleet doesn't usually make them act that sick. It is more likely to be a reproductive disorder but some photos of her, her poop and her back end would help.
What breed is she?
Can you cup your hand between her legs and feel for any swelling or bloat below her vent and/or between her legs. Do the same to healthy hens to get an idea of what normal feels like. Hens with swollen abdomens will often soil themselves. Internal swellings or masses press against the gut and constrict it and they are mostly only able to pass urates which are chalky and cake the feathers below the vent, sometimes causing irritation and burns to the skin.
Hens in this situation will sometimes develop a slow and/or sour crop, so that may be the smell or it may be an infection.
If she is not eating she will have no way of keeping warm other than burning her own body stores, which will gradually make her emaciated.... feel her breast bone to see how prominent it is.... again, compare to healthy hens. Giving her a warm bath and blow dry will sometimes make them feel better because it warms them up especially in winter. She may benefit from being placed on a heat pad.... provided she is able to get off it if she becomes too warm. If it is a reproductive ailment then usually there is little that can reasonably be done unless you have hundreds or possibly thousands of dollars to theow at the problem and it is kinder to end their suffering, but until we have more info it would be impossible to day exactly what is making her sick.