Preheat oven to 325°F. Wrap outside of springform pan with two layers of heavy-duty aluminum foil for a water bath.
Beat cream cheese and granulated sugar until smooth. Add eggs, sour cream, vanilla extract, and salt. Mix until creamy.
Pour cheesecake batter into pan, place in roasting pan. Fill with hot water halfway up sides. Bake 45-50 minutes. Chill for at least 6 hours.
Whisk all-purpose flour, granulated sugar, baking soda, and unsweetened cocoa powder in a medium bowl.
Whisk vegetable oil, buttermilk, eggs, red food coloring, white vinegar, and vanilla extract in a large bowl.
Combine wet and dry ingredients. Divide batter between two cake pans and bake at 350°F for 25-30 minutes. Let cool completely.
Cream together cream cheese and unsalted butter until light and fluffy. Slowly add powdered sugar and vanilla extract to make frosting.
Layer cake: bottom red velvet layer, a little frosting, chilled cheesecake layer, more frosting, final cake layer. Frost entire cake with crumb coat first.