Preheat oven to 325°.
Place 3/4 cup sugar in a small bowl. Scrape seeds from vanilla bean, and add seeds to sugar; discard bean. Work the vanilla seeds into sugar with fingers until well combined.
Lightly spoon flour into a dry measuring cup; level with a knife. Combine flour and sugar mixture, stirring with a whisk to combine.
Beat egg whites with a mixer at high speed until foamy. Add cream of tartar and salt; beat until soft peaks form. Add remaining 3/4 cup sugar, 2 tablespoons at a time, beating until stiff peaks form. Beat in juice. Sift flour mixture over egg white mixture, 1/4 cup at a time; fold in after each addition.
Spoon the batter into an ungreased 10-inch tube pan, spreading evenly. Break air pockets by cutting through batter with a knife. Bake at 325° for 50 minutes or until cake springs back when lightly touched. Invert pan; cool completely. Loosen cake from sides of pan using a narrow metal spatula. Invert cake onto plate.