I was in Hanoi & Saigon earlier this month. Visited Hoi An in 2014 and did a bit of cycling in its local environs. The big cities are absolute chaos, perhaps the most chaotic I've experienced along with Lahore, Pakistan. Would I do a long distance tour there? Probably not. I've grown soft and would opt for Taiwan and the ROK first. What I do want to do is to ride around Siem Reap in Cambodia and visit Angkor Wat again. If you've never been, that needs to go to the top of SEAsia, if not world travel.
Tip: you want a respite from the usual, year-round heat and humidity of the Tropics, go to Hanoi in December, January. Delightful cool, dry weather.