I've heard (and so far haven't disproven it) that the flatter ones (not so tall from top to bottom) are sweeter than the rounder ones. I'm not sure whether that's in the particular genetics of the individual onion plant, or whether the same conditions that make for growing sweeter onions also cause them to grow in a flatter shape.
I'm anxious to hear what others have to say.
I fear the mad science biology teams have improved the vidalias just like they have made other things to ship and seem fresh longer. Guess the only way to find a really good one is buy one and taste it. I quit buying them because I think they are not as good as they were ten years ago. My sweet choice now is an onion called Candy. In addition to being sweet they keep reasonably well and are easier to grow. I do not think you will find Candy onions anyplace but a farmers market.
Most vidialias these days are refrigerated. This probably breaks down the sugar and results in the taste difference. Also you may have noticed they tend to rot quickly. My neighbor brought 2 50# bags home from Georgia and lost most of them, even though placed them in a single layer on racks in his cellar.
Vidialias never were good keepers. Vidialias are a hybred so the genetics should not change.
Things that can push taste or flavor have to do with the brix level when eaten. Pushing storage life is not a friendly situation for a dead ripe Vidialia. Most likely they are being harvested before the brix level is high enough to deliver the sweet onion you remember.
Glenda, vidalias are ALWAYS more flat than round. Pervians too, and Mayans and Walla Wallas. Lots of times folks will sell texas sweets as a vidalias. Its not the same. If it aint flat instead of round, it aint sweet!
Sorry, read some posts and have an answer! If you will take the onions and gently put them in pantyhose or hose, tie a knot, add another, tie a knot, till the hose is full, gentle now, then hang them from a spot that doesnt touch anything they will last a long time. The late season vidalias are finicky because they have had their season extended, which seems to make them easier to go bad. This is the way to extend them, and one big thing! Years ago folks started making and selling tater/ onion cages. Do not use these for taters and onions! They give off a scent thats poisinious to each other, serious. We store one in the kitchen, one elsewhere. One of our rooms is a library/computer room, the onions live there. Months! If you store those two together you will get rot within ten days. Serious!