I had a similar problem, but my child threw-up in the car... someone suggested this cleaning method and it worked wonders!
Soak all the material you can with vinegar - straight vinegar, and wash down (use a sponge SOAKED with vinegar) all the plastic and other material you can't throw in a washer. Let air dry. Then go back over the car seat again with something that smells a little better (fabreeze or something similar)
After soaking the material for 15 min. wash and dry as you usually would.
This will make the car seat (& probably your car) smell like vinegar for a few days... not pleasant, but MUCH better than the smell of vomit. After that the smell fades. This was a life saver for my car! Hope it helps. Something with the vinegar that kills the bacteria in the vomit and the bacteria is what causes the lingering odor.