Prepare cake mix and bake according to package directions in a 9x13 pan.
Blend strawberries in a blender or food processor until smooth.
Add pureed strawberries, sugar, vanilla and lemon juice to a small sauce pan over medium high heat.
Heat until sugar is dissolved and strawberry sauce is thickened a bit, for about 5 minutes. Set aside.
When cake is done cooking, allow to cool for about 5 minutes.
Poke cake all over with a wooden spoon or fork.
Pour strawberry sauce over the top of the cake and spread evenly to fill all the holes.
Cover cake and allow to cool in fridge for about an hour.
Top cake with cool whip and spread evenly.
Place cake back in the fridge for another 4 hours, up to overnight.
Slice and serve.