Apparently, he tried learning it in a very short-time, and (if I recall correctly) there was a three week stint in Vietnam to achieve fluency.

I have to face it: even when my three weeks are nearly up, I can still say very little and understand less. That’s for two reasons. Firstly, I haven’t talked enough.

[...]

The second reason is that Vietnamese is difficult. Let me add one last piece of evidence. Vietnamese expresses many things in a fundamentally different way from European languages. Information is presented by means of other formulas, with other types of words. At Hanoi Central Station, a public panel with a timetable and some legal provisions isn’t captioned ‘Information’ but ‘Dear travellers ought to know’. If you say ‘I was taken to the theatre’, the correct word for ‘was’ can be ‘got’ or something like ‘endured’, depending on whether you went gladly or reluctantly. For ‘What’s the difference?’ you use ‘Where different?’ For ‘thing’, you have to choose from a whole range of options, such as ‘story’, ‘object’ or a classifier – the correct classifier, that is. Of course, there’s nothing wrong with any of these aspects of Vietnamese; they all make as much or as little sense as our own phrases and formulas.