Macerate the strawberries. In a large pot, combine the strawberries and sugar over medium heat. Stir occasionally until the strawberries release their juices and the sugar dissolves.
Add the vanilla. Split the vanilla beans lengthwise and add them to the pot. Bring the mixture to a boil, then reduce to a simmer and cook for 30 minutes.
Blend smooth. Remove the vanilla bean pods. Use an immersion blender to blend the mixture until completely smooth (or carefully transfer to a standard blender and return to the pot).
Reduce until thick. Bring back to a gentle boil, then reduce to a low simmer. Cook for 1 to 1 1/2 hours, stirring frequently, until thickened. To test doneness, run a finger across the back of a coated spoon. If the trail holds, it's ready!
Prep the jars. Wash jars, lids, and rings in hot soapy water. Keep jars hot (simmering, not boiling) in your water bath canner until ready to fill.
Add the lemon juice. Stir in the bottled lemon juice and simmer for an additional 5 minutes.
Fill the jars. Ladle hot strawberry butter into hot jars, leaving 1/4-inch headspace. Remove air bubbles, wipe rims clean, and apply lids and rings fingertip tight.
Process. Place jars in the water bath canner, covered by at least 1 inch of water. Bring to a rolling boil and process for 10 minutes, adjusting for altitude.
Cool. Turn off the heat, remove the canner lid, and let jars sit for 5 minutes. Transfer to a towel-lined surface and cool undisturbed for 12-24 hours.
Check seals and store. Check lids for a proper seal. Remove rings, label, and store in a cool, dark place for 12-18 months. Refrigerate after opening and use within 2-3 weeks.