Glad to hear your second planting prospered! How's it doing now? I'll offer my apologies up front. Normally, I wouldn't think to offer any additional comments about the situation, but, being new I need to get my post count up!!
The viability of seed year-over-year is related to some large degree by the thickness of the seed coats. Corn and soybeans are thin. Clovers, alfalfa and most other similarly sized seeds are thick and hard.
In the seed business, corn not sold this year is sent back to the processor where it's blended with "new" seed -- to raise the germination percentage. Just making up some numbers, that bag of seed corn you buy next year might have 15% last years seed and 85% seed produced just prior to the planting season. The "old" seed might have a 75% germination rate and the new seed 98%. You can do the math to calculate the percentage germination on your seed tag.
So, the old seed germination rate usually (usually) does go down. The other thing that happens is the vigor of the old seed might decrease. Germination tests are conducted in ideal conditions. When you plant old seed early in cold soils (frost!) its entirely possible for a seed in its second year to suffer serious germination problems. The same seed planted later when soils are warm and moist faces happy times!
I've not been part of the industry for a long time, but I assume the situation for soybeans remains. Few if any old soybeans are every returned to be added to new seed.
Happy day friends!