Cook the cranberries and cranberry nectar for 4-5 minutes open. Cover and refrigerate overnight.
Rinse the lime, grate the peel and squeeze the juice. Pass the raspberries through a sieve. Put cranberries with liquid, lime peel and juice, raspberry puree, kefir and vanilla sugar into a mixer and mix vigorously at highest speed.
Pour into glasses and serve garnished with lemon balm or mint.