Heat the oil in a large saucepan or soup pot over medium heat. Once it shimmers, add the bacon and cook, stirring often, until crisp, 5 to 7 minutes. If the bacon starts to get too dark, reduce the heat to medium-low.
Add the leeks and onion and cook, stirring often, until they are very soft and the leeks have reduced in volume by about half, 10 to 15 minutes. Pour in the chicken broth and vermouth, then add the potatoes and season with salt and black (or white) pepper to taste. Bring to a boil, reduce the heat to medium-low, and simmer until the potatoes are tender enough to fall apart, about 20 minutes. Let cool slightly in the pot.Working in batches, transfer the soup to the blender, never filling it more than halfway. Remove the filler cap and purée on MANUAL/LO, gradually increasing the speed to SPEED 10 (don’t fill the blender more than halfway full and always remove the filler cap of the emulsion cup when blending hot mixtures). Blend until completely smooth, about 2 minutes. Remove the emulsion cup and add 1⁄2 cup of the cream (if using). Replace the emulsion cup and select MANUAL/SPEED 2. Pulse 5 or 6 times to combine. Season with salt and black (or white) pepper to taste, and pour the soup into a clean 2-quart airtight container. Repeat with the remaining soup and cream and refrigerate until well chilled, then serve with the bell peppers, herbs, and croutons.