Put the rhubarb, pears, sugar and liqueur in a saucepan. Scrape out the vanilla pods and add both the pulp and the pods to the pot. Grate and squeeze the lime. Add about 1 teaspoon zest from the lime and the juice from the entire fruit.
Let everything simmer over low to medium heat for 20 minutes, until it is cooked together into a mass with individual pieces. If in doubt, it is better to simmer a little lower heat and a little longer (gentler on vitamins and taste, but most of the alcohol will still boil off). Let cool down. If necessary, sweeten as needed.
Spread on dessert glasses or bowls. Whip the cream until stiff and also distribute it on the glasses.