Adjust an oven rack to the middle position and preheat to 400 degrees.
Make sure the egg whites don't have even a trace of yolk in the whites as the fat in the yolk can prevent the whites from developing the volume you want.
In a small saucepan over medium-high heat, bring sugar and water to boil and cook until mixture is slightly thickened and syrupy, about 3 to 4 minutes (235 degrees on a candy thermometer). Remove from heat and cover to keep warm.
In a standing mixer fit with the whisk, or in a large (very clean) bowl with an electric mixer, whip egg whites, cream of tartar, and salt together on medium-low speed until foamy, about 1 minute.
Increase mixer speed to medium-high and whip until whites are shiny and soft peaks form, about 1 to 3 minutes.
Reduce mixer speed to medium and slowly drizzle warm syrup (avoid the whisk attachment and sides of bowl). Add vanilla, increase speed to medium-high, and whip until mixture has cooled slightly and is very thick and shiny, about 3 to 6 minutes.
Dollop the meringue evenly over the top of the warm lemon filling and spread, making sure it adheres to the crust. Use the back of a spoon to create attractive swirls and peaks.
Bake until meringue is golden brown, about 6 minutes. Transfer to a wire rack and cool until filling has set, about 2 hours. The pie can be wrapped and stored at room temperature for 1 day.
To serve, cut into wedges using a thin, sharp knife dipped into hot water between cuts. Or, hold at room temperature for up to 4 hours. The pie is best eaten the day it is baked, but it can be stored at room temperature for 1 day.