Just like while travelling to any other country, you can get international roaming activated on your number for Vietnam too, But, I'd suggest against it as it'll surely end up costing you more. So, instead reach the airport and get yourself a local Viettel SIM for about 300k VND (INR 1100) or if you can wait, you might get the same plan for cheaper in the city or at your stay. The SIM had great connectivity almost everywhere we went.
Travel to and within Vietnam
This is like the elephant in the room because when there are many options, it does get a little confusing to make a choice. So, I'll try to simplify it for you.
Travelling to Vietnam from India
> You can fly to any of Vietnam's 10+ international airports from all major Indian airports. The most popular ones are Hanoi's Noi Bai International Airport, Ho Chi Minh City's Tan Son Nhat International Airport and Da Nang's International Airport.
> There are several flight options available that include direct flights like Vietjet, Indigo, Vietnam Airlines along with Thai Airlines, Singapore airlines etc. that give you the option of connecting flights. On an average, you would pay INR 12,000 - INR 20,000 for return flights between Delhi and Hanoi/Ho Chi Minh City.
I took Indigo's connecting flight (Delhi - Kolkata - Hanoi) and then (Ho Chi Minh City - Kolkata - Delhi) for INR 22,000. We went for connecting flights because we booked just 15 days in advance so direct flights were not suitable or very expensive. So if possible, book at least a month in advance.
> I think you can also reach Vietnam by road from India, but that will definitely need an article of its own.
Travelling within Vietnam
> You can travel within Vietnam by road (taxis, train, buses) or by air (domestic flights).
According to me, the best website to book tickets for internal travel is https://12go.asia/en or the official websites of the airlines.