Combine 1/2 cup seafood seasoning, salt, beer, and vinegar in a large stockpot over high heat. Bring to a strong simmer.
Right before cooking, carefully place each crab upside down and stick a knife through the shell, just behind mouth.
Fit a screen over the beer mixture and layer the crabs on the screen. Be sure that the crabs are above the simmering liquid. Cover.
Steam crabs until they turn bright orange and all of the blue/green color is gone, 20 to 30 minutes. Sprinkle with the remaining 1/4 cup seafood seasoning before serving.
