Trim the ends from the rhubarb and cut into 15cm lengths. Soak in a large bowl full of cold water for 20 minutes. Drain and pat dry, then cut crossways into 1cm slices.
In a saucepan, stir together the orange juice and sugar. Bring to the boil and add the rhubarb. Return to the boil, then set the heat to medium-low and simmer, stirring only 2 or 3 times, until the rhubarb is tender (this should take 7 to 10 minutes - stirring too much will cause the rhubarb to break down).
Transfer the rhubarb mixture to a glass or ceramic bowl and cover with cling film. Refrigerate until cold, at least 1 hour.
Remove the seeds from the vanilla pod by cutting in half and scraping the seeds out.
Whip the cream with the icing sugar and vanilla seeds in a large bowl using a whisk until it can stand in a soft peak.
To assemble the fools, spoon 4 tablespoons of the rhubarb mixture into 4 small glasses. Cover with the same amount of whipped cream. Repeat layers until you reach the top of the glasses.
Garnish with baby mint, then cover and refrigerate for at least 1 hour (and up to 6 hours) before serving.