When the cakes are completely cold, slice each in half horizontally. Whip the cream until it holds its shape, then fold in the sugar and vanilla extract. Spread a layer of strawberry preserve over the 2 bases, then spread a layer of cream over each. Spread a layer of cream only over the remaining 2 cake halves. Sandwich 2 halves together on a serving plate, starting with a jam and cream layer, followed by a cream-only layer. Repeat the layers. Chill in the refrigerator for 2 hours before decorating.