So here's what's interesting: Hoklo pe̍h'oējī as it is commonly used, and definitely Vietnamese quốc ngũ, both maintain contrasts that don't exist in all dialects, but are generally observed in writing. So, for example, nobody ever seems to write "hànlī". Instead, they might write hànjī but pronounce it hànlī.
There is no dialect of Vietnamese that observes all the contrasts that exist in the written standard. Instead, people just read the orthography according to their own dialect(s). Sometimes, such as in karaoke in general, everybody uses the Hànội pronunciations. Other times, such as when singing the southern delta types of music (is it called quê), everybody uses the quê pronunciation.
But since POJ has come down to us mostly as an Amoy thing or an urban Taiwan thing, the system isn't equipped to deal with the central vowels. It also isn't equipped to deal with 8-tone dialects of Hoklo.
What's interesting is that the Vietnamese writing system seems almost tailor-made for this pan-Hoklo-Hokkien. We could even throw Teochew into the mix.
First, the tones. The writing system allows us six.
o ò ỏ õ ó ọ
Plus our checked tones: oh ọh (or pick any of the others)
We could also build in the distinction between the two types of 4th tones. There's the kah with a running tone that falls, and the kah with a high, even running tone.
Vowels-wise, the Vietnamese orthography is vowel-rich.
e i u o a
ê ô
ư ơ
ă â (not needed)
This system could accommodate the o / o͘ split in Hoklo as well as the two "e" sounds in Ciangciu Hoklo, not to mention the central vowels in Coânciu and Tâng'oaⁿ Hoklo. The awkward "-ere" diphthong could easily be written ơe.
That said, much of the variation is simply too complex to be captured in a writing system w/o totally sacrificing ease of learning. 間 keng vs kaiⁿ vs kuiⁿ, for example. Hànjī are just naturally strong in this area.
After all these yrs, tho, I have to say, I still haven't gotten used to the hàn-lô writing system that mixes hànjī w/ POJ. It still looks as ugly as it did at first sight.
Hangul are great b/c they mesh with hànjī. They don't really work "out of the box" b/c there's no way to indicate tones or nasalization. But what's really interesting is that the vowel inventory also seems to be almost tailor made for "pan-Hoklo". Here they are with their equivalents in POJ and quoc ngu:
ㅣ
i
i/y
ㅡ
ir
ư
ㅜ
u
u
ㅗ
o
ô
ㅓ
er
ơ
ㅔ
e
ê
ㅐ
e (open e, with the dot)
e
ㅏ
a
Missing: no open o; some diphthongs aren't available, such as -iau, which would have to be written using 얄 -ial or some similar compromise.
So'í, nạ sỉ bơh iọng Oạtlảm ể "kokgứjị" (KGJ) lải siá Họkló bủn, i ệ sêⁿ cồ ánnê. Kồh ụ cịt ể hốchù: tữ nuíthế hongbịn, KGJ kỉsịt pí POJ khah hố chúlí.
Cit cụn sịtcãi sĩ siuⁿ ẻng 'a.
That just sounds funny to (at least) TW ears.