In a large saucepan combine the pork, 1 of the yellow or white onions, sliced, the vinegar, salt to taste, and enough water to cover the mixture by 2 inches, bring the water to a boil, and simmer the mixture, covered partially, for 1 1/2 to 2 hours, or until the pork is tender. Let the pork cool in the broth, transfer it to a work surface, reserving 3/4 cup of the broth, and with forks shred it. In a large heavy skillet cook the remaining yellow or white onion, chopped fine, and the garlic in 1 tablespoon of the oil over moderately low heat, stirring, until the onion is softened and stir in the pork, the reserved broth, and salt and pepper to taste. The pork mixture may be made 1 day in advance and kept covered and chilled. In a large saucepan, cleaned, combine the corn with cold water to cover, bring the water to a boil, and simmer the corn for 1 minute. Drain the corn, let it cool until it can be handled, and, working over a bowl, with a serrated knife cut the kernels from the cobs. To the bowl add the tomatoes, the red onion, the jalapenos, the coriander, 2 tablespoons of the lime juice, the remaining 2 tablespoons oil, and salt to taste, stir the salsa until it is combined, and let it stand for 30 minutes. The salsa may be made 4 hours in advance and kept covered and chilled. Peel and pit the avocados, in a bowl mash them with a fork, and stir in the remaining 2 tablespoons lime juice and salt to taste. In a skillet heat 1/2 inch of the additional oil over moderately high heat until it is hot but not smoking and in it fry the tortillas, 1 at a time, for 1 minute, or until they are golden, transferring them as they are fried to paper towels to drain. Spread the avocado mixture on the tortillas, mound the pork mixture on it, and spoon the salsa over the pork.