It was my first time in Vietnam, I visited Ho Chi Minh City and Nha Trang. Overall everything was great! At least not as hot as in Thailand! It is clear that the country is developing rapidly! Lots of things are being built! You have an interesting fleet of vehicles, different from neighboring countries. It's a shame that I had only 5 days... hope to come back again in the future for a longer time.
You're right that we have an interesting fleet of vehicles. So many endemic car brands like Thaco, Vinaxuki, Vinfast and Dothanh, as well as too many endemic motorbike brands too, which I wouldn't list here.
Also, did you spot any GAZ vans in HCMC? Last time I went there (two weeks ago), I saw many of them.
Anyway, per Chinese trucks, I would estimate about one BIG (i.e., tractor units and the like) one for every five Vietnamese or Japanese equivalents. I had the opportunity to go to Thủ Đức Agromarket in the northeast of Saigon, and it's a gold mine for platespotters. Go there next time if you don't mind the smell.