Preheat the oven to 450°F. In a heavy Dutch oven or large (4-quart) saucepan bring the broth and milk to a simmer over medium-high heat.
Reduce the heat, sprinkle in the polenta, whisking all the while, and keep adjusting the heat so that the mixture stays at a gentle simmer. Season with salt and pepper. Continue to simmer and whisk semi-frequently until the mixture has thickened and become creamy, about 20 minutes.
While the polenta is cooking, heat 2 tablespoons of the oil in a large skillet, preferably nonstick, over medium heat. Add the shallot and sauté for 2 minutes, then add the tomatoes and oregano and sauté for another 4 minutes or so, until the tomatoes have collapsed slightly. Transfer the tomatoes to a bowl, stir in the vinegar, and tent with foil to keep warm.
Return the pan to medium high heat and heat the remaining tablespoon of oil. Season the salmon with salt and pepper. Place the salmon skin side down (if applicable) in the hot pan and sear for 4 minutes. Transfer the pan to the oven and roast the fish for about 5 minutes, until cooked to your liking.
Meanwhile, when the polenta has finished cooking, remove it from the heat and whisk in the Parmesan and the butter. Taste and add pepper and additional salt, if needed. Divide the polenta among 4 plates. Use a metal spatula to transfer the salmon filets on top of the polenta, and spoon over some of the tomato vinaigrette. Serve hot.