Combine sugar, salt, syrup, and water in a heavy saucepan. Cook over medium heat, stirring frequently, until mixture comes to a boil and sugar is dissolved. Continue cooking until the mixture reaches soft ball stage (240°).
Beat egg whites (at room temperature) until foamy. While beating at medium speed of electric mixer, slowly pour one-third of hot syrup in a thin stream over egg whites.
Cook remaining two-thirds of syrup to firm ball stage (248°). Slowly pour remaining syrup over egg white mixture while beating at medium speed of electric mixer. Turn mixer to high speed, and continue beating until stiff peaks form and frosting is thick enough to spread. Stir in vanilla and pecans. Spread immediately on cooled cake.