Preheat the oven to 375 degrees F. Set the vol-au-vent on a non-stick baking sheet. Bake 20 minutes, or until puffed and golden. (These can be baked several hours in advance. When needed, reheat in the oven a few minutes.)
Heat the oil in wide skillet set over medium-high. Add prawns and cook 1 minute per side. Lift prawns out of the skillet and set on a plate. Now cook the scallops 1 minute per side, and then set on the plate with the prawns.
Lower the heat to medium. Add the shallot and garlic to the skillet and cook 1 minute. Add the wine, bring to a simmer and reduce by half. Add the stock/flour mixture, paprika, tarragon and cayenne pepper. Bring to a simmer, and simmer 2 minutes. Add the salmon to the sauce, return to a simmer, and simmer 2 minutes.
Mix in the prawns and scallops and cook 2 to 3 minutes more, or until all seafood is cooked through. Stir in the cream, taste the sauce and season with salt, if needed.
Set a vol-au-vent on each of six plates. Cut out a circle pastry in the centre of each vol-au-vent to create a cavity. Spoon the seafood mixture in and around the vol-au-vent. Sprinkle with parsley and serve.
Note: If desired, you could top the vol-au-vent, once filled, with the circle of pastry you cut out to create the cavity.