Line an 8-by-11-inch baking pan with foil. Spray the foil with vegetable oil. In a heavy saucepan, melt the butter. Stir in the sugar and water and bring to a boil. Wash down the side of the pan with a moistened pastry brush. Cook over moderate heat, stirring with a wooden spoon, until a deeply golden caramel forms and the temperature reaches 300° on a candy thermometer, 15 minutes; if the sugar and butter separate, stir vigorously to blend. Remove from the heat and add the coarsely chopped almonds, vanilla and salt. Scrape the toffee into the prepared pan; let cool for 10 minutes.