Combine 2 tablespoons olive oil and the vinegar. Add the sliced tomatoes and toss gently to coat them. Set aside.
Slice the avocadoes and sprinkle with the lemon juice to keep them from turning brown. Set aside.
Heat a griddle or cast iron skillet over medium heat or preheat the broiler to high. Combine the remaining olive oil with the garlic. Season with salt and pepper.
Cut the foccacia in half horizontally and brush with the garlic oil mixture. Griddle or broil, oiled sided facing down, until toasted, about 4 minutes.
Spread the mayonnaise on the toasted foccacia and then layer the sandwich filling on top of the bread: lettuce leaves, sliced tomatoes, bacon, and finally, sliced avocado. Close the sandwich and serve at once.