Denied Boarding with 144 hour transit-free visa

It would be useful if you could present us your detail itinerary. At the first sight, your itinerary:

Mumbai - Vietnam (Ho Chi Minh, Hanoi) - Shanghai - Taiwan

If these are all your stops within your itinerary, it is definitely and undeniably eligible for 144 hour visa free transit if your entire transit time in Shanghai was longer than 24 hours and for 24 hour visa free transit if your entire transit time was shorter than 24 hours.

Now, it is hard to say why were you denied boarding because it was perfectly enough if they have just looked at Timatic, that is the ultimate tool for any kind of visa and similar requirements, and there they would have clearly seen that you were eligible to travel absolutely visa free. Here I have to point out again, if your itinerary is as I have understood it and if you have properly presented it. In any case, I hope we will hear from you with additional details. Thank you.

India is not one of the countries qualified for 144-hour transit-free visa. Only 6 Asian countries are: Korea, Japan, Singapore, Brunei, UAE and Qatar.

So I'm a U.S. citizen, I was just visiting family in India and hence decided to fly from India (for extra info I'm an Overseas Citizen)

My itinerary was Mumbai - Hanoi (layover) - Shanghai - Taipei. I didn't have a visa to entire Vietnam, but I'm not sure if that matters because it was just a layover (there are not direct flights to Shanghai from Mumbai, but maybe I could have avoided it if I had flown to Hong Kong?).

I'm also not sure if they had looked at Timatic, they were just trying to contact supervisors and they were confused that this was even an option.

Could you confirm your scheduled arrival date in Shanghai please? 31st December should have been ok, but 30th would not be.

You should have been allowed entry as an American citizen. I printed up the program declaration from the China Southern website to show to the counter staff. It's the local (non-China) counter staff who are unaware of the program. Once you get into China proper, there's no issue so long as your departing ticket is within 144 hours and to a third country not from where you just left.

Yup, was landing on the 31st, so I think it should have been fine to leave on the 6th

Indeed, a really weird situation. Based on your itinerary, dates of travel, citizenship, you should have been allowed to proceed to accomplish your journey because you were absolutely in accordance with 144 hour visa free transit rules. Now, after our short discussion I can only conclude that the check-in staff has shown a huge dose of incompetence and arrogance because if only they have checked Timatic, they would be informed that you were good to go. Unfortunately, this example just shows how certain people and their incompetence can cause huge problems to those how are in perfect compliance with every single rule and regulation. The worst thing is their refusal to take a look at Timatic. Timatic exists exactly for this purpose because nobody can be informed about every single regulation that exists in air transport and relevant border, visa and other formalities. I am sorry for your loss of money and time and I just hope that you will write an email to Vietnam Airlines in detail and ask from them a compensation because they owe it to you. In any case, I wish you all the best in your future travels :)

The important thing is to make sure that check-in staff put China as a transit in Timatic, and not the destination. If they put China as the destination then it will say that a visa is required. Only if they put China as the transit will they see the transit-without-visa rules.