Acetaldehyde is normal in beer that is still fermenting as it is an intermediate step for the yeast on the journey from wort to beer. People mostly describe it as green apple or cidery but some people perceive it as vinegar. It would be very unusual for you to get vinegar in the primary because the bacteria that produce it are aerobic, they need oxygen to survive and your beer should be covered in CO2. You can get it in secondary if you have a wide surface on your beer because there will only be the dissolved CO2 to provide the blanket and that may not be enough. That's why it's recommended to use a smaller secondary and fill it to the neck so you limit the space for the CO2 to fill.
