You are absolutely right about car-vomiting being due to "nerves," rather than motion sickness. And as you may already know, medication won't cure this sort of thing: it requires a behavioral approach. Start out feeding your dog in the parked car, in a secure, contained area. Don't urge her to go in: just allow her to, if hungry. Keep this up for around a week, or until you notice her happily entering your parked car. Next, when she is dining, turn on the ignition, but don't drive. Keep it on for around 5 minutes. Let her decide whether to continue dining, or not. Keep this up by increments of a few minutes at a time. If she shows confidence, build to just pulling in and out of your garage/driveway. Eventually, you'll be able to take her for trips around the block, and finally, on to freeways!