Which prime lens + 16-80 for Vietnam, Cambodia and Thailand

... regarding primes as I do portraits and details.

But it appears that Fuji shooters that have some credibility here seem to prefer zooms.

I am neither a credible Fuji shooter nor particularly well liked here so take my opinion as just that: an opinion. Nothing more.

To start with: why would anybody mention the 16-55/2.8 when your question was about how to complement your 16-80/4? Extreme wide angles might also not what you are after, Halong Bay might be more 24mm (FF than 19mm)

Since you used a Pen-F, here is a shot taken with the 12/2 (24mm, like your 16mm equivalent:

Wider focal lengths can make islands a bit smallish. You might or might not like it. But if you do, Eric's suggestion of the 13/1.4 is a good one. Not for portraits though. I shoot people and for that you might want to consider a light tele, even the 35/1.4 is a great choice, depending on how good your communication skills are with people when you get real close.

One of the most fabulous places to do people is Ta Phrom, like Angkor Wat, near Siem Reap (pronounced see-am ree-app, not sy-am etc) and Angkor Wat is only one of the temples you can visit. Angkor Wat isn't even the largest. But you will find out. Good fun plus a cultural experience quite unique in the world.

Fuji 90/2 (135mm full frame)

Olympus 75/1.8 (150mm full frame)

Another Pen-F since you are familiar with that ... 75/1.8 150mm again full frame...

Fuji 90/2 again, Hanoi street

Another 90mm

Or 50/2 a fantastic lens

Fuji 50/2.0

I always liked to go close, go intimate ... have been to SEA many, many times. If I could only take one lens it would possibly be the 50/2. MFD also excellent!

Fuji 50/2

Good luck with your trip!

Deed