What domestic airlines are good in Vietnam?

Don't know Hahn Air but we fly Vietjet often and it's fine.  Same par with Jetstar.  Checked bag is not included but we've rarely ever needed it so it's not an issue to us.  We always pay a few dollars more for 2 aisle seats on the first standard row so it's comfortable all the way.

From Saigon to Da Nang, yes on flying.  From Saigon to Nha Trang, we prefer the train because Cam Ranh Airport is 30 km away while you can walk from the train station to the centre.

Take the day train for scenery (the sea is on the right hand side facing forward) and night train to save time.

PS:  You didn't ask about Vietnam Airlines, but here's a little something, just in case:  Everyone I know who flies with VA refuses to switch to Vietjet and Jetstar, even people (the locals) with limited travel budget.  VA has great customer service, good reward program, and very good on time record. 

They're not a LCC, however.

I flew round trip from Saigon to Hanoi last year. To Hanoi, I was on Vietnam Airlines which was comparable to Eva Air for international. Service was decent and you get drinks and snack or maybe a meal. On the way back, I was on VietJet and it was not the same. First, I had to show them the credit card I used to book the flight. VN Airlines didn't ask for the credit card and I didn't have it with me because my US wallet was in Saigon and wasn't planning to use a credit card. I had to pay for the flight in cash (part USD and part Dong) or else they would not check me in which they did refund to my credit card about a month later. The flight was bare bones, no drinks or snacks. I believe they rescheduled the flight a couple times on me too. If you can avoid VietJet, I would say avoid it but if you don't have a choice, guess it's better than train.

Hi qnbui,
Vietjet are a LCC..low cost carrier.
You need to compare apples to apples.    With a LCC, & in life ,  you get what you pay for.
If you want a nice seat upfront or exit row.......you pay extra
Food, drinks, etc ....you pay .
Extra baggage ....you pay

And it's usually reflected in the ticket prices.

Thanks everyone, some of my internal flights are only for one hour, think a LCC will be fine. I am booking 6 flights so just trying to save a little money. I have used Jetstar many times in and out of Australia and they are a LCC too.

I think you'll be fine. It's not as if your spending 12 hours on the bird.
From memory and experience none have recently dropped out of the sky or had issues. Sit back and enjoy the serenity cuz.