Useful, but it doesn't tell me how long 'tết' lasts. Also unsure if I should avoid "People Power" day.
Avoid Easter (easy if you're going in February!). People Power is just one day; if you follow the link, it claims that schools may be out, but businesses stay open, so you might get a one day surge of tourists.
What I did: I looked at my work and personal calendar (including a very late last minute mandatory family trip at the end of April). I then reached out to a resort (you can do more than one) and ask for a no-obligation proposal for hotel room, diving, and meals. Finally, I looked up flight costs, and committed the bookings. For me, it will be 10 days of diving in May. (7 is too short, 14 will put too much wear and tear on me, and I don't like packing, moving, unpacking, and diving again).
El Galleon hotel and Asia Divers gets good reviews from SB members. There are a several other resorts with good reviews; I've heard positive reviews of La Laguna Beach & Dive Center, and there are SB member reviews of Scandi Divers (where I wound up booking).
To misquote a popular commercial, just do it.
Hope to see a great trip report (with lots of photos) from you before I leave on my own trip!