In a medium saucepan, combine the berries, sugar, lemon juice and salt.
Place the pan over medium heat and bring to a gentle simmer. Reduce the heat to medium low and cook, stirring often, until the berries break down and the jam thickens slightly, about 10-15 minutes. It will continue to thicken as it cools.
Remove from heat and stir in vanilla extract.
Let cool and transfer to a clean glass jar. Cover and refrigerate. Use within 2 to 3 weeks. You can also freeze small portions for longer storage.
Notes
Don't be tempted to overcook the jam; it will continue to thicken as it cools.