For crust, in a large bowl, cream butter and sugar until light and fluffy. Beat in vanilla. Gradually add the flour, oats and salt and mix well. Press over the bottom and sides of a 9-in. pie plate. Cover and refrigerate.
Combine rhubarb and water in a saucepan. Bring to a boil, stirring constantly. Combine the sugar, flour and salt; add to the rhubarb mixture. Cook and stir over medium-high heat until thickened and bubbly. Reduce heat; cook and stir 2 minutes longer. Remove from the heat. Stir a small amount of hot filling into egg yolks; return all to pan, stirring constantly. Bring to a gentle boil; cook and stir 2 minutes longer. Remove from the heat. Cool slightly.
Pour filling into crust. Bake at 375° for 25-30 minutes.
In a large bowl, beat egg whites and cream of tartar on medium speed until soft peaks form. Gradually beat in sugar, 1 tablespoon at a time, on high until stiff glossy peaks form and sugar is dissolved. Beat in vanilla. Spread evenly over hot filling, sealing edges to crust.
Bake at 350° for 12-15 minutes or until the meringue is golden brown. Cool on a wire rack for 1 hour. Refrigerate for at least 3 hours before serving. Store leftovers in the refrigerator.