That's a feature, not a bug! Or so says:

http://www.cjvlang.com/Writing/writsys/writviet.html

In spite of its shortcomings, the system that the missionaries created was remarkably suited to the Vietnamese language. Of particular usefulness is its ability to bridge dialects. The writing system tries to show not only distinctions in sound that are found in the standard Hanoi dialect but also those in other dialects. Two different letters may be pronounced identically in Hanoi but differently in other dialects.

For me, the biggest problem with Đ is that it's far more common than simple D.
In General, Max Pinton is right, Vietnamese spelling is more like German or Polish orthography, a compromise dialect worked out over generations meant to encompass as many local differences as possible. Not perfect, but preferable to the alternative (according to all the Vietnamese I know).

It is a bit heavy on diacritics and could jettison some (such as the high tone marker in syllables that end in -p, -t, -ch, -c) with no ill effect. But for me the bigger problem is that the unit of writing is primarily the syllable rather than the word (thanks a lot, Chinese influence!)

"No self-respecting revolutionary (aside from Mao) uses his or her birth name"

Huh?

Sorry, Fidel! It's just that, you know, you're stuck there on that little island and it's easy to forget you... (Actually, I know that's not the real Fidel, because he would have left a comment that took six hours to read.)

As for the writing system, yes, of course it's a good idea to have symbols that can bridge dialects -- that's not what I'm complaining about. What I'm complaining about is the fact that for two different phonemes they used two characters that are bound to be printed the same in any text that doesn't emanate from Vietnam, so that it's impossible without special expertise to know that the two d's in Ngo Dinh Diem are pronounced very differently -- and, as Michael Ferris says, it makes no sense that the "plain" d is used for the less-common z/y phoneme.

(Goddammit, I had to rewrite that comment to avoid the word "spec1alist" because my spam-blocker noticed it contained the banned word "c1alis." [Replace 1 by i, obviously.] Hoist by my own petard!)

"What I'm complaining about is the fact that for two different phonemes they used two characters that are bound to be printed the same in any text that doesn't emanate from Vietnam"

Why is the lone consontant example worse than the several vowel examples of the same phenomenon? Not counting tones, the following vowel phonemes (distinguished in all dialects I'm aware of) have the same problem

a also â, ă
o also ô, ơ
e also ê
u also ư

Sure, the d just happened to be what was on my mind.

I have a peculiar relationship with Vietnamese. When I meet people who speak an interesting language, I always try to sit down with them and learn a bit about it. Because I've studied some phonetics, I can usually get cheap praise from my informants: "Hey! You said that very well! Most Americans can't make those sounds!" Stroke stroke purr.

But not Vietnamese. "No. Listen again. Now try. No, that's still not right. Listen." I have never managed to pronounce a Vietnamese sentence correctly. In fact, I have never managed to pronounce the name Nguyen correctly. And this wasn't the opinion of one picky informant: these discouraging judgements came from a series of Vietnamese-speakers encountered at intervals over three decades. Vietnamese, she is hard.

As for that barred D: it isn't anything close to "like an English d"; our host has clearly never run his version past a native speaker. As near as I can tell, it is ingressive. Or glottalized. Or perhaps pronounced with a coarticulation that is only possible if you have a Vietnamese uvula.

(Now your blog will be #1 on Google for "Vietnamese uvula." Never say I never did anything for you.)

Yes, Viet phonology is truly fiendish (hardest by far of any I've tackled). I can sympathize with the no sentence pronounced correctly syndrome (I actually do get occasional mild praise but it's clear that I'm still far from acceptable).
The Đ is prevoiced (Thompson analized it as /?t/ (preglottalized /t/ with concomitant voicing, the B is similar (Thompson has it as /?p/ (but not G, strangely enough).

our host has clearly never run his version past a native speaker.

Absolutely correct, and I don't actually have a version -- all I meant was that, unlike the unbarred "d," it bore a remote resemblance to what you'd think it might signify -- dental stop and all that. I'm grateful for the more detailed information.

To pronounce Vietnamese đ (barred d), I just use the Japanese flapped r (as in ラ 'ra'). It seems to work.

Wow, you guys seem to be having difficulty....of how to pronounce D, or the bar with the D and other letters with accents on top.
Good luck though.
Ba can be father or dad...Ba can also mean Mrs...or something like that. :D
Have fun!