Microwave at 50% power in 15-second intervals, just until softened, not melted.
In a separate mixing bowl, combine the sweetened condensed milk, lemon juice, and vanilla extract and blend until the mixture is smooth. Ensure the mixture is fully smooth before adding cream cheese.
Add the softened cream cheese to the bowl and blend again until the filling is completely smooth and creamy.
Pour the lemon filling into the prepared graham cracker crust and spread evenly.
Refrigerate the pie for at least 1 hour for a soft set, or 3–4 hours for clean slices.
Serve when the filling is fully set.