Trim off and discard the thick stems from the collard greens. In batches, stack the leaves and cut crosswise into strips about ½ inch wide. Fill the sink or a large bowl with cold water. Add the greens and swirl them to loosen any grit. Transfer the collard greens, with any water clinging to them, to a large bowl.
In a large pot or a large, deep sauté pan, fry the bacon in the oil over medium heat until browned and crisp, about 8 minutes. Using a slotted spoon, transfer the bacon to paper towels to drain. Remove the pot from the heat, with the bacon fat still in it, and let cool slightly.
Return the pot to medium-low heat. Add the garlic and cook, stirring frequently, until softened, about 1 minute. Increase the heat to medium-high. Add a handful of the collard greens to the pot, cover, and cook until they wilt. Continue adding the greens, a handful at a time and allowing them to wilt before adding the next batch, until all of the greens are in the pot. Add the red pepper flakes, cover, reduce the heat to medium-low, and simmer, stirring occasionally, for about 15 minutes if you like the greens tender but still fresh, or up to 45 minutes if you like them well done.
Stir in the reserved bacon and the 2 tablespoons vinegar. Season with salt, then taste and adjust with red pepper flakes. Transfer the greens and their cooking liquid to a warmed serving bowl. Serve hot, passing vinegar on the side.