If they sneeze out of the house then it's nothing to do with your house, but if they only sneeze in the house then suspect some problem in the house. Is it too dusty, do you sprinkle Diatom or other dust and have the chickens access to scrat about in dusty litter? Do you use hay as litter if so, don't it breeds mould spores. Only use softwood shavings, paper, or any sterile non dusty and absorbent material intended for animal bedding, or if chopped straw (don't get it wet and change regularly on a weekly to fortnightly basis depending on whether the hens have access to it).
Whether your ventillation is adequate or not depends on the size of your house and the number of birds. It would do no harm to make the holes wider than you indicate as they seem too small. Cover with mesh if you are concerned about rodents entering. Don't make it too drafty though. You'll have to use your judgement.
If they are sneezing out of the house too and have runny noses and or discharge from their eyes keep them under observation and if it gets worse it could be a respiratory problem and could need treatment with medication and you may need the services of a vet.
Don't know what effect ACV will have on the cause of sneezing but probably none. If you do want to use it, and it won't do any harm, then use the unrefined ACV intended for an animal tonic not the supermarket refined type. It's all the cloudy stuff they leave in which is claimed to deliver the benefit. Giving them an odd apple or two to peck at might just do as well.
Regards