If there is any doubt, I would go ahead and treat. The vaccine helps prevent severe outbreaks, but is not 100% effective, so there can still sometimes be illness. Since half are not vaccinated, those birds would be at higher risk. If all are acting ok, you can wait and see, but I would be keeping a very close eye on them, any lethargy at all, acting off at all, would be concerning, and then I would definitely treat. Coccidiosis can kill, and sometimes it can be quick. Blood on roosts can be from a pecking injury in a spat, not uncommon. Combs and wattles can bleed a lot, and afterwards you may see nothing but a tiny black spot on the comb or wattle, which can look like nothing that could cause the amount of blood you saw. I had a roo get a nicked comb, he shook his head and blood went everywhere. The coop looked like a massacre had happened. He was fine.