Arrange fish in a single layer in bottom of a 10-inch skillet; add vermouth, capers, and clam juice. Bring to a simmer; cover and simmer over medium-low heat 10 minutes or until fish flakes easily when tested with a fork. Remove fish from pan, and keep warm; reserve cooking liquid. Bring cooking liquid to a boil; cook until reduced by half (about 9 minutes). Gradually add hot liquid to egg yolk in a bowl, stirring constantly with a whisk. Return egg mixture to pan. Add mayonnaise, milk, and red pepper; cook over medium heat 1 minute, stirring constantly. Stir in parsley. Spoon 3 tablespoons sauce onto each of 4 plates; top each portion with 2 fillets and 2 toast slices.