Do I need multiple entry visa for flight from ho chi minh to New Delhi (3-hour transit in Hanoi)

Dear Mel B, 

Thanks for your questions, 

You have stamp for 21 days staying Vietnam and it's still valid, is it correct?

If yes, you can use it to fly to Ha Noi from Ho Chi Minh without any problem. you don't need to apply for a multiple visa 

Thanks and best regards, 

Camellia ( ms )