Key point: Is the smell in the leather, or is it in the stitching and leechs through the stitch holes? Further, I believe I read that a lot of automotive leather is ventilated (full of holes) even if it doesn't obviously appear this way. That means the vomit went THROUGH, and this isn't specifically a leather problem, but your solution needs to be safe for leather.
Either way, I'm sorry that your car smells like vomit.
