For mobile coverage and speed, I've found Japan (I've been from Ishigaki to Shiretoko in the past few years) to be pretty world class (assuming your device supports the proper bands - for anyone visiting, refer to https://www.frequencycheck.com/countries/japan ). For wired connections, almost every new building in Tokyo has fiber, and 10Gbps support is pretty common (NTT, Sony, Softbank and others all offer it) while I don't believe it's widely available at all in Vietnam.

Anyway, everyone will have their own experience, and I haven't been in Vietnam since before the pandemic, but last time I was there (4 cities and a few out of town trips), while I had OK connectivity, I wasn't blown away by it vs Japan or other places. I'd take Japanese traffic safety and street sanitation over Vietnam every day of the year though.