We spent just under a month in Vietnam recently.
Overall we found it just ok. The worst, for us, is the noise. Particularly the Viet obsession with scooter horns. And the cities in Vietnam are not particularly walkable because moving (and constantly beeping) scooters fill the streets and parked scooters fill the sidewalks leaving nowhere to actually walk.
We really did NOT love Hanoi and only spent a few days there. So loud. So unwalkable. Our views on Hanoi:
https://readyjetroam.com/2019/03/12/...ot-love-hanoi/
We were in Da Nang for about 3 weeks. But we were there on slow travel working on our business, not touring. It's an interesting city to get to know...but in the way, say, Pittsburgh is an interesting city: it's really just a city...not much to 'see.' The beach is ok, but we only ever went to it once...and we lived 4 blocks from it! It's not really set up for spending a lot of time on. But it's a cheap place to be so I understand your desire to base out of there. My advice is: don't. Stay where you want to be. Lodging is cheap all over Vietnam if you are willing to leave western hotels behind. Take advantage of it!
We spent a couple days in Hoi An. I agree with the assessment that it's a tourist town...then again, we were there as tourists so it made for a lovely couple of day despite the throngs of tourists (of which we were two!). The restaurants were awesome. The beach at An Bang was an added bonus. Our report on two days in Hoi An:
https://readyjetroam.com/2019/03/20/two-days-in-hoi-an/
Have fun! We didn't love Vietnam the way we love Thailand and parts of Malaysia, but it's an experience we'll never forget!