In a medium saucepan, combine strawberries, sugar and vanilla. Cook over medium-high heat stirring constantly until mixture comes to a boil. Reduce heat and continue cooking for 5 minutes, stirring occasionally until sauce starts to thicken. Mash strawberries as you stir.
Remove from heat and stir in lemon juice.
Allow to cool before serving. Store in airtight container in refrigerator for up to 7 days.
When the strawberries start to boil, the mixture will be frothy. Keep stirring and the froth will gradually disappear.