Special equipment: a 5-cup gelatin mold
For the cranberry layer: In a medium saucepan set over medium-high heat, add 1 3/4 cups of the cranberry juice, the sugar, allspice, cinnamon and orange zest, and bring to a boil. Remove from the heat and infuse the spiced cranberry juice for 10 minutes. Add the gelatin to the remaining 3/4 cup cranberry juice to dissolve. Strain the infused cranberry juice into the gelatin mixture and whisk to combine. Pour into the gelatin mold and sprinkle with the pomegranate seeds. Refrigerate until set, at least 6 hours or up to overnight.
For the vanilla layer: In a small saucepan set over medium heat, add 1/4 cup of the milk, the sugar and scraped vanilla seeds. Heat until warmed but not boiling. In the meantime, dissolve the gelatin in the remaining 3/4 cup milk. While whisking, strain the warm milk into the gelatin mixture. Then whisk in the sour cream. Cool to room temperature before pouring the cream over the cranberry layer. Refrigerate until set, at least 6 hours or up to overnight.
For the orange layer: Dissolve the gelatin in the boiling water, whisking until fully dissolved. Add 1 cup cold water. Cool to room temperature before pouring the orange gelatin mixture over the vanilla layer. Sprinkle the orange segments into the mold and refrigerate until set, at least 6 hours or up to overnight.
To unmold, dip the mold in hot water to loosen. Invert and serve.