I have been a strict 'heirloom only' gardener for my whole life. However, the other day I was reading about tomato hybridization and it said that in F1 hybrids there is often the characteristic of 'hybrid vigor'. This article said that F1 exhibits a stronger and more productive plant, but it fades away by F2 and by F3 it is non-existent. This caused me to think of a possibility. I have some tomato seeds and herb seeds that are from different countries, but they are the same variety.
For instance, I have seeds for the Brandywine tomato from Canada, Germany, South Carolina, and Oregon. They are all the same variety, but they have been selectively bred in different climates and have been totally isolated from each other for untold decades; in all probability. It would stand to reason that they are not precisely the same genetics, although they should be very close.
So, the question is: if I crossed the Canadian version with the German version, would the resulting seeds exhibit increased vigor?
A second question is: since these are not technically hybrids, would the vigor remain in the next generations F2, F3, etc instead of diminishing as hybrids will?
"So, the question is: if I crossed the Canadian version with the German version, would the resulting seeds exhibit increased vigor?"
They could, because the Canadian version and the German version may have been inbred for enough generations for different minor mutations to exist in each. Even though they share the same name, they could be, in effect, different strains.
"A second question is: since these are not technically hybrids, would the vigor remain in the next generations F2, F3, etc instead of diminishing as hybrids will?"
The vigor would not diminish automatically, but it would tend to diminish unless you carefully selected just the very best specimens to save seeds from. The key to plant breeding of any kind is to select the very best specimen or specimens to save seeds from. That also applies to saving seeds from heirloom varieties that have not been cross-pollinated. Pick the very best specimen or specimens that best present the characteristics expected from that heirloom variety.
And be aware that it is a numbers game. It is one thing to pick the best specimen from a planting of a dozen plants. It is an entirely different thing to pick the best specimen from a planting of a thousand plants. You are not looking for the biggest, reddest, best tasting tomato. You are looking for the same thing that the judges of a state fair would be looking for. Are the tomatoes the best representatives of that specific variety? If you are a plant breeder hoping to create an improved new variety, then the rules of the game are entirely different. But regardless of which "game" you are playing, your odds for success increase with the number of plants that you can grow.