With the rack in the middle position, preheat the oven to 400°F (200°C). Line a baking sheet with parchment paper.
Place the slices of pancetta side by side on the baking sheet. Bake for 12 to 15 minutes, or until crisp. Set aside on paper towels. Adjust the oven to broil.
In a pot of simmering salted water, add the vinegar. Crack the eggs one at a time in a saucer and slide each into the water as you go.Let poach for about 3 minutes or until the desired doneness. The yolks should be runny. Set the poached eggs on a plate to drain.
On the same baking sheet, without parchment paper, place the slices of bread. Broil. Turn the slices over. Sprinkle four with the cheese. Continue baking for about 5 minutes or until the cheese is golden brown and the bread is toasted.
In a bowl, whisk together the oil, vinegar and mustard. Add the lettuce and toss to coat. Season with salt and pepper.
Layer the pancetta over the slices of cheese toast. Add the eggs and salad. Spread the mayonnaise on the remaining toast and close the sandwiches.