In a stand mixer, whip cream cheese and sugar together until smooth. Stop the mixer and, using a spatula, scrape the bowl periodically. Gradually, add the heavy cream and vanilla and continue whipping. Again, periodically stopping the mixer and scraping the bowl to pull all the cream cheese into the heavy cream. Continue to whip at a high speed until all combined and light and fluffy. Spread whipped cream cheese topping over the chilled lemon filling. Garnish with thin lemon slices, if desired.