Sometimes it is sweat, breath, or poo that can smell like vinegar in newborn babies. Additionally, if your baby has been drinking milk that has made its way into your baby’s neck folds, you could begin to notice that your baby smells particularly sour. To prevent this, always wash your baby’s neck folds, and try to bathe your baby regularly to keep strange smells emitting from the body at bay. Vinegar smells coming from a baby’s breath may be indicative of something more serious, so be sure to seek a doctor’s advice.