How to Make It
In a food processor, combine the vinegar, mustard and shallot and process until smooth. With the machine on, slowly add the olive and vegetable oils and process until emulsified. Add the thyme and tarragon and pulse just until combined. Transfer to a bowl and season with salt and pepper.
In a large pot of boiling salted water, cook the yellow wax and green beans until crisp-tender, about 7 minutes. Using a slotted spoon, transfer the beans to a colander and refresh under cold water. Drain and pat dry. Add the limas to the pot and boil until tender, about 4 minutes. Drain and refresh under cold water. Drain well and pat dry.
In a large skillet, heat the olive oil until shimmering. Add the corn and cook over moderate heat, stirring often, until crisp-tender, about 3 minutes. Transfer the corn to a plate to cool.
In a very large bowl, combine the yellow wax and green beans with the lima beans, corn, tomatoes and scallions. Add the dressing and toss to coat. Season with salt and pepper and serve.