Get some poultry dust for the vent area. YOu have to keep it maggot-free. And keep the area very clean. Maggots don't go onto healthy animals in general. So there may be something else going on before you saw them. Keep her indoors if possible, keep her clean, and make sure she's eating. If she's egg-bound, you'll have to deal with that also.
If you go to a feed store and ask for a vitamin and electrolyte powder to add to water they will know. I think there is one called chick saver. I use Avia Charge but you have to order it online as far as I know. But the packets at the feed store are cheeper. Can you see any other injuries around her vent?
And yes your other chickens can drink it.But I would keep her seperate until you know what is wrong and she is healthy.
In addition to the other suggestions, I would suggest getting some GSE into her. Another great thing to bath them in is MMS. You can purchase it on Amazon. I have bathed many a chicken (and dog, kids, me) in it with amazing results. It is also very affordable and will kill anything bad without harming the good so it is very effective. It can also be taken internally and I have given it internally to chickens, dogs, goats, and humans with amazing results as well.