Wash the cherries, remove the stems, and pit them. Place them in a large pot.
Slice open the vanilla pod and scrape the seeds into the cherries. Add the empty pod to the pot as well.
Mix everything with the gelling sugar and a splash of lemon juice.
Let the mixture sit for 2 hours to allow the sugar to dissolve and the juice to release from the cherries.
If you prefer a smoother jam, briefly purée the fruits now; otherwise, leave everything as it is. Place the pot on the stove, heat and bring the mixture to a boil while stirring.Boil the cherries for 4 to 5 minutes on high heat until they start to thicken up.
Perform a consistency test: Place a bit of cherry jam on a cold plate. If it sets immediately, forming a fine skin, it's done. If not, let it cook a minute longer and repeat testing.
Remove the vanilla pod and pour the still-hot cherry jam into clean jars. Seal them immediately and let cool.