Why Some Languages Sound Faster Than Others

I don't speak Vietnamese, so can't comment directly with that, but I can relate to my experiences in watching public Australian English simplify massively over the past 20 years or so. The wider public take a lot of cues from the media. Soundbite journalism be damned.

People who learn Chinese often fret about getting the tones right. The tones just aren't that important - Chinese speakers can generally guess the meaning, though they will think you sound like a 4-year-old if you don't pronounce tones correctly. IMO, getting the vowels and consonants right is harder (and more important).

Seriously, here's the pairs you will confuse:

d / t - d is unaspirated

j / zh - j is a "cjsch" sound (a bit like "Asia") while zh is a "j" sound

q / ch - q is a "bright" (slightly whistled?) ch; ch is a "dark" ch)

x / sh - x is a "bright" sh and sh is a "dark" sh

c / s - c is a "ts", s is just s

b / p - b can sound a little closer to p than in English and p is more aspirated

g / k - g sounds a little close to k, while k is more aspirated

Then there's the vowels, which are really hard. Learning four tones is comparatively easy.

If you don't get the consonants almost 100% correct, people will simply not be able to tell what you are saying. If you don't use tones, they can usually understand, as long as you use simple words (which face it, you will).

I suspect it's related to the extremely strict and ridged hierarchies in China, and the extremely flexible and implicit social networks. Your boss doesn't ask you to do a favor. They order you. In contrast, you don't ask your neighbour if you can borrow a cup or sugar, you just take it, and pay back the favor some other time. If they don't let you (and don't have a good reason), you just never talk to them again (or if that's too severe, silently downgrade your relationship with them).

Funnily, "let" and "make" are the same word in Chinese. i.e. "My mother made me do my homework, then she made me watch TV".

The study of Linguistics, explicitly, does not deal with orthography, or the written system of languages. There are of course exceptions with good reasons, but orthography systems are rarely, if ever, good representations of the systems of auditory communication that are formally considered languages. An orthography system can be heavily influenced by geopolitics (Chinese), have severe ambiguities (Arabic, the various Latin alphabet systems), or have been created retroactively (many languages of indigenous peoples). While it is convenient to map a spoken language to its related orthography when discussing topics such as syllables, inflection, and morphology, it is rarely appropriate when studying linguistics formally.

As for Japanese, while it is true that its alphabet system has a relatively straightforward mapping to its phonology, the mapping itself is, unfortunately, not unambiguous. Japanese has a tonal system[1] that is not explicit in its orthography. There are examples of phonemically distinct words that are identical when written in hiragana/katakana.

Finally, there exists a moraic system[2] which sits between the phonemic and syllabic abstractions. Japanese, especially, have many phenomenons that cannot be adequately modelled unless working in this in-between system.

[1]: http://en.wikipedia.org/wiki/Japanese_pitch_accent

[2]: http://en.wikipedia.org/wiki/Mora_%28linguistics%29