In a bowl, mix limoncello, lemon juice, water, and sugar until sugar dissolves.
Whip heavy cream until soft peaks form. In another bowl, beat mascarpone, powdered sugar, lemon zest, and vanilla until smooth. Gently fold whipped cream into mascarpone mixture.
Quickly dip each ladyfinger into the lemon syrup — avoid soaking too long.
Arrange soaked ladyfingers in a dish. Spread half of mascarpone cream over top. Repeat layers.
Cover and refrigerate at least 4 hours or overnight.
Sprinkle fresh lemon zest before serving.