I don't add anything unless they need it. Electrolytes don't do much for you except rehydrate you faster if you're dehydrated. If you're not dehydrated, then it's about the same as drinking regular water (has to do with salt transport in your kidneys.) And honestly, I think it has too much salt to be healthy for chickens on a regular basis. Chickens aren't supposed to have a lot of salt. It's not like they do a lot of sweating.
As for vitamins, do you think your chickens are deficient in anything? Do you feed a decent quality feed? If no and yes, respectively, I don't think vitamins are going to do anything for it.
In this case, it sounds like they need more shade, or perhaps a fan with an icebucket in front of it. There's an area under our porch that's just dry dirt. It's in the shade, and they dig themselves a hole in the cool earth and sleep there.