Put the vermicelli in a bowl. Bring a pot of lightly salted water to a boil, and pour over the vermicelli. Soak for 4 minutes, until tender. Drain and rinse under cold water. Cut the vermicelli into 1-inch lengths.
In a saucepan, combine the rhubarb with the water, sugar, and salt. Bring to a boil, stirring to dissolve the sugar. Cover and lower the heat to a simmer. Cook for about 10 minutes, until the rhubarb is very soft. Let cool completely.
Pour the rhubarb into a blender. Add the rose water and lime juice and blend until smooth. Stir the vermicelli and rhubarb together in a large bowl. Pour into a shallow baking dish and freeze, uncovered.
Rake the faloodeh with a fork after 2 hours to prevent it from freezing into a solid mass. Freeze and stir again after 2 hours. Repeat after another 2 hours, if necessary. The faloodeh will have a malleable consistency, somewhere between ice cream and sorbet, within 4-6 hours. Scoop it into serving bowls and top with pistachios and lime juice. Drizzle with sour cherry syrup, if desired. The faloodeh tastes best within 24 hours of being made.