China 24h visa free

Yes, it is. Your itinerary:

Vietnam - Shanghai (I assume that had Shanghai in mind when you have written Xangai) - Paris

If you do not have other landing(s) and if your entire transit time in Shanghai is shorter than 24 hours, you will be covered by 24 hour visa free transit and you do not need any kind of visa.

Yes, you and your wife should be eligible for the 24-hour visa free. Just make sure you have the required documents.

Indeed, you are advised to print out your flight tickets and also your hotel booking confirmation if you plan to stay overnight in a hotel, outside of the airport.