I have taken the first tentative steps in to Vietnamese.
The tones are interesting. There are six (in the Hanoi dialect), compared to Thai's five. Three of them seem to roughly match Thai's mid, rising, and falling tones. The other three, however, include glottal stops! Very interesting.
The tones are quite easy to write. Thai has a very elegant and insane method to mark tones, involving three classes of consonants, live and dead syllables, long and short vowels, and tone markers. Vietnamese does it much more simply: Each tone has a diacritic, and it is placed over (or in one case, under) the vowel of the syllable.
I'm not sure how to approach the writing system. Tiếng Việt is written in a modified Latin alphabet (like English), and it's very difficult to scan without subconsciously pronouncing it as I do English. I haven't had this problem since Latin class in high school. I do have a vague understanding of the differing orthography of some European languages- but this comes from familiarity with loan words. Vietnamese's phonology is unlike any European language's (or, apparently, Thai's.) So it being written in a Latin script is almost the opposite of helpful.
I need to somehow suppress my instincts until I am comfortable with the spoken language. I should probably seek out an audio-only course of some sort. Teach Yourself Vietnamese has a nice exposition of tones and phonetics, but it's a lot to take in.
Last time I started studying a new language (Arabic), I quickly realized how much Thai I actually already knew, and decided to put my effort in to that language. I hope I don't do that this time, but starting from scratch is honestly a bit daunting.