Vietnamese language models not working?

Hi - has anyone gotten the Vietnamese language models to work accurately?

I have tried the models in English, Chinese and French and they all work fine with relatively low error rates.

But for some reason Vietnamese seems completely broken. I tried the same sentences with Google speech recognition and I can get fairly high accuracy.

I also tried to include a grammar to restrict the words but it only returns [unk] for nearly everything, or completely unrelated (but Vietnamese) words. Could something be broken with the model itself?