Vidalias are a very sweet onion and have a very short shelf life. You need to get them as fresh as possible. They tend to go bad in just a few weeks. Some folks refer to them as weed onions. Mine tend to never last that long as I use them as a staple along with garlic and potatoes.
Apparently there are several varities of onions that are just as sweet such as some from California.
Paul E. Smith