Flight Rome - Chengdu - Ho Chi Minh City (Vietnamese passport)

The most important detail is that you will travel through China (Chengdu in your case) covered by 24 hour visa free transit. So, you will not need any kind of visa.

This visa free scheme allows you to travel visa free via Chengdu and there is no doubt about it. Both of your journeys are eligible for visa free transit scheme and there is no difference between outward and return flights. Having in mind that you plan to exit the airport in order to enjoy some time in the city, you will approach 24/144 hour visa free transit counter immediately upon your landing in Chengdu. There you will apply for a temporary entry permit that is issued to those with long layovers, but always based on discretionary decision of airport authorities on a day of travel. You will present your flight itinerary and even better if you can add accommodation booking confirmation in case that you decide to rest in some of local accommodations. Since your layovers are long, I would say that your chances to be granted a temporary entry permit are very good. This application for a temporary entry permit is free of any charge.