Hi!,
I was there in April/May this year. Only a week but we went HCMC, Hanoi, Hoi An, HCMC. We loved it and never saw a tunnel! Unfortunately my wife enjoyed it more than Bali which was a blow for me, I connect more with the Balinese and the surroundings in Ubud, north etc.
I would thoroughly recommend all three places I visited but I think Hanoi was the more interesting of the two large centres.
Please ask me any questions you have.
Cheers,
David