Cut the rhubarb into small pieces. Trim the strawberries and cut them into halves.
Set a large 7-8 quart saucepot over medium heat. Add the strawberries, rhubarb, sugar, lemon juice, and salt.
Bring to a boil, then lower the heat to a very low boil and cook for one hour. Make sure to stir the jam every 5 to 10 minutes so that it doesn’t burn on the bottom.
Once the jam has cooked for at least an hour, place a cooking thermometer in the pot. Wait for the jam to reach 203 to 205°F. The jam may need up to another 30 minutes to reach this temperature, or it may already be there!
Turn off the heat and stir in the vanilla extract. Use a ladle to carefully move the hot jam to four clean pint jars.
If planning to freeze, make sure to leave 1 inch at the top of the jar unfilled, for expansion. If planning to can, follow proper canning protocol at this point.