Set up a steamer for the crabs. The steamer insert should sit 2 to 3 inches above the bottom of the pot so that the crabs do not touch the liquid beneath.
Add 1 1/2 cups water, lager, vinegar and 2 tablespoons seafood seasoning to the bottom of the pot. Whisk to combine and bring to a simmer.
Layer the crabs in the steamer insert, generously sprinkling the salt and remaining seafood seasoning on each layer of crabs.
Cover the pot and steam the crabs for 30 minutes. The cooked crabs should be a bright orange color throughout with no trace of blue or green.