The choice of one onion over another is really going to come down to personal preference based on color and flavor.
Red and white onions are usually milder in flavor than yellow onions which is the reason they're often the choice for hamburgers and sandwiches.
Yellow (sometimes referred to as "Spanish") onions tend to have a more pungent flavor.
Sweet onions (Vidalia, Maui, Walla-Walla, Texas 1015) develop more sweetness and fewer sulfuric compounds due to the mineral content of the soil that they're grown in. This is why you can have onion sets (small bulbs) from any of these varieties but they won't necessarily be sweet (or as sweet) because a lot of it has to do with the makeup of the soiling they're grown in.
Two years ago I hosted Jeanne Jones (syndicated writer of "Cook it Light" column for King Features syndicate and writer of over 33 cookbooks including "Canyon Ranch Spa Cookbook") for guest chef cooking classes. If I recall correctly, she said she used primarily white onions because they have a lower sodium content than any other type.
Did that "mean black eyed pea salad" recipe happen to come from a certain "Low Country Cooking Class"? If so, I used it for two reasons: not everyone will have access to Vidalia onions and it provides additional color.
Escallion = scallion = green onion.
I think it's possible to substitute regular onions in some situations, but there are a lot of variables.
First off, scallions have a sharper, more grassy flavor than onions, though they're not as strong.
Then there's the question of which kind of onion you want to substitute. White onions have some of that same sharp flavor, and are quite strong. Yellow onions are much milder and sweeter, and might not be such a close flavor match. Red onions are somewhere in between, in my experience. I would probably try white onion, but in much smaller quantity than called for of scallions.
It's also important to consider how the scallions are being used. If they're being cooked into the recipe, that's very different than if they're being added right at the end, or even as a garnish. I think the substitution is less of an issue if the scallions are to be cooked. If they're to be more or less raw, I'd suggest white onion, not too much of it, and sliced very very thinly, as a raw white onion is really strong.
Whatever you try, bear in mind that it won't be exactly the same. But that certainly doesn't mean it'll be bad--just different.
Best Answer
As Wikipedia states (with good sources):
As far as I understand, there are varieties with puprlish tint in the mix. The most defining thing about onion being accepted as Vidalia is taste, capability of growing well and not developing sharp taste when grown on low sulfur soil around Vidalia, Georgia. If taste is sweet and mellow as you would expect it to, then color is probably OK and onion is within what state of Georgia allows to call an vidalia onion.
It is also possible that what you got was unplanned crossbreed with red onions from near field. This can happen if onion is grown from seeds, especially seeds gathered by the farmer himself, as seed selling companies tend to be very careful about that. In that case, these onions should not be sold as Vidalia onion - but if they are half-Vidalia-accepted-variety and grown in proper soil, taste should be pretty close to what you paid for.