Preheat the oven to 350 degrees. Spray a 9x13 pan with nonstick cooking spray and set aside.
Prepare the strawberry cake mix according to the package instructions, add lemon zest and vanilla. Pour the batter into the pan and bake for 25-30 minutes, or until a skewer/knife comes out cleanly. Let cool for 10 minutes.
To make the lemonade syrup, combine lemon juice, water and sugar in a small saucepan and simmer until sugar dissolves. Remove from heat and stir in the butter. Let cool slightly.
Use a skewer or the back of a spoon handle to poke holes all over the cake. Pour the lemonade syrup evenly over the cake. Let it absorb while cooling completely.
In a large bowl, beat the cream cheese, powdered sugar and vanilla until smooth.
In a separate bowl, whip the heavy cream to stiff peaks.
Gently fold the whipped cream into the cream cheese mixture until fluffy.
Combine the diced strawberries, sugar and lemon juice and lemon zest in a medium sized bowl.
Assemble the cake by spreading the whipped cream cheese filling evenly over the cooled cake.
Use a slotted spoon or drain the strawberries before spooning over the top of the cake.
Refrigerate at least 2 hours before serving.