Combine the beer, vinegar, Old Bay, and salt in a medium nonreactive bowl; set aside.
Place a rack in the bottom of a large, deep, nonreactive kettle.
Pile half the crabs on the rack and pour in half the beer mixture; add the remaining crabs and beer mixture.
Set over med-high heat and bring to a boil; adjust the heat so the liquid bubbles gently, cover tight, and steam the crabs for 20-25 minutes or until they are bright red.
Serve at once with melted butter and plenty of napkins.
Don't forget to put out a large bowl to catch the crab shells.