In a 2 quart saucepan, heat the vegetable oil. Add the pork chunks and cook over medium heat until well browned, about 15 to 20 minutes. Remove from heat.
Heat the olive oil in a large 12-inch skillet. Add the chopped onions, celery, bell pepper, tomatoes and garlic, sauté for 5 minutes.
Once the vegetables are well cooked, process them briefly in a food processor until a puree.
Return the puree to the skillet. Add the vinegar and oregano and season to taste with salt and pepper. Add 3 cups of water, cover, and simmer for 10 minutes.
Roughly chop the cooked pork in the food processor, mincing into 1/2-inch pieces.
Add the pork to the sofrito, simmer covered for 10 minutes. Remove from the heat.
Using a slotted spoon, remove the cooked meat from the pan, and place in a clean bow, add filling ingredients. Reserve the cooking liquid in the pan to make the masa, pouring it into a large glass measuring cup.
Add enough water to the reserved cooking liquid to equal six cups of liquid total.
In a separate bowl, prepare the masa by pouring in the harina de maiz. Add the reserved cooking liquid/water one cup at a time, stirring constantly to avoid lumps.
Add the salt. The masa will need to be the consistency of mashed potatoes. If mixture is too lumpy, use an electric hand mixer to help break up the lumps.
Once masa is the right consistency, stir in the corn kernels.
Preheat your oven to 350°F. Using a bit of olive oil, grease the interior of a 3 1/2 quart casserole dish.
Pour half of the masa mixture into the casserole, spreading it across the entire bottom of the pan in an even layer.
Spread all of the meat mixture in an even layer on top of the masa layer, then add the remaining masa, making sure the masa reaches all edges of the casserole.
Cover with foil and bake in the oven for 50 minutes. Remove the foil and bake for another 10 minutes.
Remove from the oven and cool briefly before serving.