Could Vietnamese use tonal spelling akin to something like Gwoyeu?
Hmong, which has 7 or 8 tones, is spoken in Vietnam, so a similar tone-letter system for Vietnamese is feasible, but not politically realistic. The sentence Son mwngz hwnj max gvaq dah (Son m hn ma gva da) "Teach thee to climb on a horse to cross a river" is often used to help people remember the six tones. Zhuang, the languages spoken by millions in in Guangxi Zhuang Autonomous Region, is another possible model. The idea that Gwoyeu Romatzyh amounts to weird random letters tacked onto Mandarin syllables is an unfortunate misconception propagated by people who have not bothered to look into GRs design principles. Starting from an initial draft, GR Tonal Spelling was carefully refined to (1) work in parallel to Bopomofo (both leave first tone unmarked) (2) minimize clutter caused by superfluous letters (GR leaves second tone MNLR syllables unmarked). Whats more, the letters have mnemonic value as reminders of tone contours. Designers of a possible tonal orthography for Vietnamese would be advised to study GRs design principles. However, I dont think anybody but language nerds would ever think of creating such a spelling system (to facilitate language learning): If it aint broke, dont fix it! is the operating principle.