Hi,

According to my experience, many students said Vietnamese was difficult because it is a tonal language. If you already know other tonal language like Chinese, it would be easier to learn. Also, because of that, you need to make it right at the beginning or it is quite hard to correct your pronunciation later. 

Luckily, Vietnamese grammar is not difficult. You do not need to conjugate the verb with the pronouns and the tenses, no gender like French. However, you need to build up your vocab  and make it pronounced right. This is the important thing for learning any languages. Basically, you need to build up vocab and know how to use it in a sentence, because Vietnamese has many words with the same pronunciation but different meanings depending on the situations.

There are some basic lessons for learning Vietnamese online. You can search those on Youtube. For the complete beginner, you also need to choose the dialect to learn Vietnamese because the words are used and pronounced differently in 2 main dialects: Nothern accent and Southern accent. (North: Ha Noi, South: Ho Chi Minh). 

So in general, if you want to learn Vietnamese, go for the dialect first. Which one you should choose is depending on the place you gonna go in Vietnam or your own purpose. Maybe, a lot of your Vietnamese friends speak Southern accent? Then, go for the alphabet and the tone. Try to make it right. Listen and practice with the locals. Build up vocab and put them into practice for long-term memory. 

OK. That's all. I hope that helps you. If you are interested in having a professional teacher, you can have a look at my profile. 
Have a nice day and I hope that you enjoy your learning Vietnamese. 

All the best,

Nga

I don't speak Vietnamese, but I do have experience studying a tonal language that's totally disimilar from my native language, so I would guess a lot of my experience still applies.

i would guess that the pronunciation would be enormously difficult for the first month, or two, or three. Vietnamese has a lot of sounds that don't exist in English or Spanish, and while tones are not actually /that/ hard to produce, they take a lot of practice to master. That said, Vietnamese pronunciation is consistent, as the writing is phonetic, so once you've got it, you've got it and won't need to worry about it. 

the vocabulary will probably be very hard to learn at first, because if your target language isn't tonal, it's hard to think of tones as an intrinsic part of a word. After you've learned a few hundred words, though, it'll get easier.

The grammar will be very different, but not necessarily hard. I'm pretty sure Vietnamese is non-inflective, which means there's less to memorize, but also means that it works very differently from Spanish.

Basically; expect it to be really hard for the first few months, but totally manageable after that. A lot of work,  sure, but all languages are a lot of work.