How much planning and booking do I need to do? Coming in 47 days

I am landing in HCMC on the 21st Jul and will my flight back is 5th Aug.

General question is: How much do I need to plan day to day and specifically book things in advance?

I am used to planning very carefully for family holidays and in Europe in general. Also for myself, I like to have a good idea of what I am going to visit and see - gets me excited.

Do I need to book though?

When I travelled in Thailand back in 2008 and Indonesia in 2012, no booking was necessary.

From the airports there, the cab drivers simply drove me to Soi Rambutri in Bangkok, or Jalan Jaksa in Jakarta (backpacker hotspots), respectively and that was it. I always found accommodation immediately on the spot.

Meanwhile, I had a fairly detailed itinerary and covered a lot in both cases.

Can I operate like that in Vietnam or should I book in advance?

FYI, I will be solo backpacking, my travel style is to hop around and experience the country as much as possible: sights, sea and beaches, hikes and nature (chill, no Herculean efforts necessary). Get a glimpse of authentic life, probably avoid overhyped, super touristy places (already know I will skip Danang).

I am still forming my itinerary highlights and it would be useful to know if I need to completely fix them right now and start booking immediately or I can leave it more open and play it by ear when I get there.

My current idea is cover HCMC+Mekong delta to ca.middle of the country (Hue) with stops on the way and then back. But completely open. I might decide to fly to Hanoi...or to Pompenh or Siem Reap.

I will finalize my target itinerary this week and need to know how final and firm it needs to be - will come back with a question about good bases/stops then.