Dredge scallops in 3 tsp of the cornflour, shaking off the excess. Heat oil in a large non-stick frying pan over medium heat. Add scallops and sauté until golden brown and cooked through, about 3 minutes. Using a slotted spoon, transfer the scallops to a bowl.
Add garlic to pan; cook 1 minute. Add tomatoes and cook until they begin to collapse, about 4 minutes. Add vermouth, wine or stock, the salt and the basil. Bring to the boil; cook for about 1 minute.
Blend remaining cornflour and cold water in a small bowl. Add mixture to pan and cook, stirring, until sauce is slightly thickened, about 1 minute.
Return scallops to pan. Reduce to a simmer; cook just until heated through, about 1 minute.