If you can speak Cantonese, learning Vietnamese is much easier, because some of the pronunciations are exactly the same. When I was in Vietnam and talking with my mum in Cantonese, Vietnamese people could catch a bit what we said. Sometimes when you listen carefully, they seem to be talking a mixture of Cantonese and Hakka.
Actually Vietnamese is actually Chinese in hanyu pinyin form. Few hundred years ago, Vietnamese was written in Chinese characters. During the 19th century, when French priest came to spread Christianity in Vietnam, one of them romanized the Vietnamese words into today's present form.
Look deep into it, Vietnamese culture is actually about 70-80% Chinese culture. They may be originated from China, of course they won't admit, in fact they came out with some kind of fairy tales about their existence, which when I read it, is quite laughable.
Vietnamese is not that difficult to learn, pronunciations have few tones like in Chinese, because its foundation is actually Chinese. Easier than learning Japanese. But the language sounds crude or rough, unlike Thai, which is quite nice language, like singing.
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