Combine graham crumbs and butter and press firmly into the bottom of an 8" or 9" springform pan and about 1" up the sides.
In a large bowl, beat cream cheese with an electric mixer until smooth.
Add 1 1/2 cups sugar, lime juice, lime zest and vanilla and beat until smooth.
Whip the cream in a separate bowl with an electric mixer until stiff peaks form. Fold the whipped cream into the cream cheese mixture carefully so you do not deflate it. Taste and adjust the sweetness if necessary.
Spread into prepared pan, cover and refrigerate for at least 5-6 hours or until firm. Slice and serve with sweetened whipped cream as desired.