You can import Ralf's Vietnamese dictionary (version 0.1; GPLv3) into simon. The dictionary contains about 6.000 words; training is not possible. The phoneme elements contain eSpeak phonemes (not IPA phonemes).
Tags: eSpeak, GPLv3, PLS, tjə̌ˀŋ vjə̀t, vi
There is a discussion at Voxforge.