Preheat oven to 225ºF. Line 2 large baking sheets with parchment. Place egg whites in a clean, dry bowl. Using clean, dry beaters, beat on low speed, slowly bringing it up to medium speed, until whites are frothy. Add lemon juice and beat, slowly raising speed to medium-high, until whites turn glossy. Sift in sweetener and continue to beat until firm peaks form. Fold in salt and vanilla. Transfer to a piping bag fitted with a plain tip, or spoon into a large ziplock bag and snip off a corner. Pipe into 1 1/2-inch rounds on baking sheets. (Alternatively, use 2 spoons to form meringues.) Bake for 25 to 30 minutes, until firm and just beginning to brown slightly. Turn off oven, prop open with a wooden spoon and let cool in the oven for 1 hour. Peel off parchment and serve. Store leftovers in an airtight container at room temperature.