Start with the berries: rinse them well, hull them, and cut them into quarters, and put them in a large heavy-bottomed pot. (I used my favorite 4 qt pot, which was perfect for the task.) Scrape the pulp from the vanilla beans, then add that and the beans themselves to the berries. Add the sugar and stir well. Cover with a clean tea towel, and let stand 1 to 2 hours to bring the juice out of the berries. (You can let them stand, covered, in the refrigerator for up to 2 days, if you need to.)
Meanwhile, prepare jars and any other equipment. Make sure your jars are sparkling clean. Wash the lids in warm water and set aside. Keep the bands handy. Have an extra small jar, or custard cup, clean and ready for any excess jam. Put a couple of small saucers in the freezer to chill so they’ll be ready for testing the jam.
Add the lemon juice and a tiny bit of butter to the reserved pot of strawberries and sugar. (The butter is optional, but it does reduce foaming) Put the pot over medium heat and bring to a boil, stirring occasionally. When the berries are completely cooked, briefly remove the pot from the heat. Remove and discard the vanilla pods. (Hint: you could rinse the pods, then store them in a little tin of sugar, to end up with vanilla sugar)Whiz the preserves with an immersion blender to make a smooth purée.
Return the preserves to the heat, and continue cooking, stirring often, until it bubbles and looks quite thick. I like to test for doneness by putting a spoonful on a saucer, and chilling it in the refrigerator for 2 minutes. After that time, if I can drag my finger through the jam, and leave a clear track that doesn’t fill in again, it’s ready. In my kitchen, this took about 10 minutes more. Remove from heat.
When the jam is ready, remove the pot from the heat. Ladle jam into prepared jars, wipe rims, and apply lids and bands. Anything that won’t fit in your four jars can go into the clean custard cup and chill it in the fridge.
Process for 10 minutes in a boiling water bath or steam canner. Cool jars, label, and store in a dark cool place.