Robert,
Excellent question!!
I also bought some Vidalia Onion seeds last week (Yellow Granex Hybrid 33) with confusing planting instructions.
I found a Vidalia Onion Growers website which may be of help to you:
http://www.vidaliaonion.org/farming/...vidalia_onions
Basically, in Georgia growers there start their seeds in September, with transplant out in November - January. The key is to amend your soil to have as low a Sulfur content as possible to get a similar taste. Here is another link of interest:
http://www.gardenersnet.com/vegetable/vidaliaonion.htm
I've started 32 seeds last week just to observe seedling habits, then will be starting seedlings in September for real. Hope to then transplant out in December.
As you are in a similar latitude to Vidalia Georgia, you may want to follow their dates.
Raybo