Something about "How is the rainfall here" strikes me as strange in English. I tend to expect that a "how is" question should be answerable with "good, bad or indifferent" or, less likely, with a description such as, "It comes down in torrents." "Precipitation" instead of "rainfall" might call for a more technical response. I guess it is a question of context and register. If the question were "The soil is good here but how is the rainfall?" I'd know what was meant. Maybe the same applies to Vietnamese. Just musing.
You can ask "what is the rainfall here?" or "how much is the rainfall here?" but you wouldn't ask "HOW" is the rainfall, unless it were some kind of convoluted conversation. Please add to the acceptable answers.
In the English that I am familiar with when asking about rainfall 'what's the rainfall here' is commonly used, to which the answer is commonly an amount, e.g.1200 mm, or the equivalent in words, e.g. 'a lot'. The translated example answer 'how is the rainfall here' is rarely used and would be a little confusing.