Do not beat yourself too much. A big mistake was made, it happens. As you did not add any fish and you had them for at least 5 month, it is not a disease, but it is related to the adding of the sump. Now that your purple fish is still alive, with water change it most probably will survive.
Now before adding the sump again, clean it thoroughly with bleach. Then rinse, rinse, and rinse some more before adding Prime or other dechlorinating product.
Adding a sump should be beneficial, as it adds water volume, it just happen it either was not cleaned enough, or too acidic due to the vinegar. Adding denitryfiying bacteria would help too.
Good luck.