First, bring the sugar and scraped vanilla pulp with water to a boil, stirring, and cook until the sugar has dissolved.
Put in a suitable bowl with the rhubarb and cover. Place in a warm water bath and let steep for 75 minutes.
Line a strainer with fine cotton cloth (gauze) and drain the rhubarb, reserving the juice.
Squeeze the rhubarb well in the tea towel. Season juice with orange blossom water and lemon juice.
Pour the syrup into well-cleaned bottles and seal immediately.