Preheat the oven to 400°F (200°C).
Heat a sauté pan over medium heat. Add the pancetta (or bacon) and cook until it begins to crisp up a little. Add the onions and celery and sauté for about 5 minutes, until the onions look translucent. Add the sage, thyme and garlic and sauté 1 minute more. Pour in the vermouth and simmer until the liquid reduces by half.
Place the cubed bread in a bowl. Add the sautéed pancetta mixture and the parsley and toss well. Season to taste.
Spoon the stuffing into a roasting pan and divide it into two sections (a chicken half will sit on top of each portion of stuffing).
Use a sharp butcher’s knife or kitchen shears to split the chicken in half by cutting down either side of the backbone, with the backbone resting on your cutting board. The backbone can be saved for stock. Flip the chicken over to remove the keel bone and cartilage between the two breasts. Run your knife around either side of the keel bone at the base of the cartilage and then use your fingers or a smaller knife to run along the cartilage to loosen it. Use your finger to pop out both the keel bone and cartilage, which you can save or discard. Split the chicken in half between the two breasts and place one half on each portion of stuffing, skin side up.
Lightly brush or drizzle the chicken with oil, sprinkle with paprika and season with salt and pepper.
Roast the chicken, uncovered, for about an hour, until an internal temperature of 165°F (74°C) is reached when tested inside the thigh and centre of the breast. Serve the chicken on the bone, cutting into smaller pieces, if needed, with the stuffing on the side.
Note: Leftover chicken and stuffing will keep, well wrapped, in the fridge for up to 2 days.