I don't have cider vinegar. Could I substitute buttermilk for the milk and vinegar, or does the vinegar serve another purpose?

When I made it, I didn't have any cider vinegar in my house either (or milk, for that matter, so I subbed half & half). Instead, I had rice vinegar. The resulting cake came out pretty tasty. Unless Amanda can tell us if the cider vinegar is essential, I'd say go with your substitution if that's all you've got. I figure that for the teaspoon the recipe calls for, the change won't cause too much chaos to the final product.

Yes you can use buttermilk. The vinegar adds acid as does the buttermilk, to react with the baking soda.