Scrape all the seeds from the vanilla bean and add to the cream. Add the sugar, basil, lemon zest, and salt. Cover over medium-low heat for 5 minutes.
Remove from the heat, and set aside for 30 minutes. Remove the vanilla bean.
Strain the mixture, through a fine strainer, into a clean bowl. Discard the rosemary and zest.
Place in a bowl, cool and place in refrigerator.