1. Combine first 8 ingredients in a small pan over medium-low heat. Cook, stirring occasionally, 3 minutes or until butter melts. Keep warm.
2. Combine 6 quarts water, vinegar, and next 3 ingredients in a large stockpot. Bring to a boil over high heat.
3. While water is heating, set crabs on a rimmed baking sheet or tray, and put in freezer. (This makes handling the crabs easier--they should be well chilled but not frozen; don't leave them in freezer more than 30 minutes.) When water has come to a rolling boil, grab each crab securely at the back of its shell, and gently but swiftly drop it headfirst into the boiling water.
4. Cover pot, and return water to a boil. Cook Dungeness crabs 18 to 20 minutes and blue crabs 5 minutes. Drain, clean, and serve with warm butter sauce.