Vietnamese is spoken by some 80 million people in Vietnam and about 2 million people overseas. Vietnamese has 3 main dialects: northern, central, and southern.
As Vietnamese has a Latin-based Alphabet, the pronunciation of the unaccented characters is not difficult. However, it's a tonal language with diacritics so things get difficult here. But with some efforts and practice, you'll soon get used to it.
Vietnamese grammar is relatively simple in terms of tenses. There is no conjugation or inflection. The main sentence structure is subject + verb + object.
Vietnamese use the Latin a-z alphabet, and a couple of other characters based on this set (eg: ă).
As such, writing would be quite familiar to latin-based language natives
Common difficulties Edit
The Foreign Service Institute has classified Vietnamese as a "Hard" language. It is estimated that learning Vietnamese to a Professional Working Proficiency in the language (a score of Speaking-3/Reading-3 on the Interagency Language Roundtable scale) will take an average of 44 weeks (1100 class hours).
Vietnamese is a tonal language so it usually takes some times for people whose native language is non-tonal.
In fact, as with any languages, there are many elements that constitute a true native accent, that without an extensive period of practice time and attentive attitude, one can never master (The case being immigrants who live in the US for tens of years, without acquiring a reasonable command of the English language, not to mention American accent). Fortunately, the Vietnamese people are so tolerant and appreciative to foreigners who put some efforts into learning Vietnamese, that they don't care about how badly the foreigners sound. Therefore, a lot of foreigners are considered fluent in the language, but in reality their pronunciation is far from Vietnamese, let alone intonation, or rythm. In short, difficulty is quite relative in this case, especially if you are a Westerner (from Europe, the US, Canada, Australia, and other English speaking country), then learning enough Vietnamese so people start appereciating you is easy.
There is an Foreign Service Institute course for Vietnamese.
This site (Seasite Edu) is another useful and free course with dialogue and audio.
This Learn Vietnamese Your Way site is the another very useful site with grammar, vocabulary and other resources.
Pimsleur offers a course in Vietnamese.
Duolingo offers a course in Vietnamese.
Rosetta Stone offers a course in Vietnamese
It should be noted that the majority of Vietnamese courses available internationally are teaching southern Vietnamese accent, but there is another dominant accent from the North (Hanoi - the capital). So students who have learnt the southern one may have some initial trouble if they are to communicate with a nothern accent user.