I'm very glad they are doing better!
The vaccine or medicated feed can help reduce the risk of outbreaks in chicks, but neither is 100% effective. The vaccine is usually applied by droplets sprayed on chicks, which they then ingest by pecking the droplets on each other, so it's possible that a chick can get missed. With both medicated feed or vaccination, they only work if the chicks are exposed to the coccidia.
If you get vaccinated chicks, then do not feed medicated feed as it will negate the vaccine. Either way, vaccinated or medicated feed, if you have an outbreak you still have to treat with full strength medication at that point. The medicated feed contains a very low dose of amprolium, the same medication in Corid, intended to keep the numbers low enough to allow them to build resistance. If the numbers of coccidia become too great, the birds get sick and you have to treat with the full strength medication.
For chicks in a brooder, I start on day one providing them with soil from my yard in a plastic plant saucer every day. This exposes them naturally to all the microbes in the soil while their immune systems are developing and gives them a chance to build resistance naturally , just like if they were being raised by a broody hen outside. The saucer makes it easy to dump and clean and give new soil every day. They love it and scratch and peck and dirt bathe in it, dirt party! Since doing it this way I have not had a major outbreak, 5 or 6 years now. I always have Corid on hand just in case, so I can treat quickly if necessary.
For length of treatment, if they seem a little slow to recover, then go the full 7 days. If at the end of 7 days if you still have any doubts, if any one of them seems to be slower to recover you can do another 5 - 7 days at 1/3 tsp of the powder per gallon of water, or 1/2 tsp of the liquid per gallon. If I have to treat, then once treatment is complete then I generally will give vitamins and probiotics for a few days following the end of treatment, to help get their guts back in shape. Just don't give vitamins during treatment. Giving thiamine during treatment can reduce the effectiveness of the medication.