Yeah, the letter system that's in use was introduced by Jesuit missionaries. You can read more about it here. It's nice because it shows the reader which tones to use. Prior to the current alphabet, Vietnam used a script derived from Chinese characters, which shouldn't be surprising, given its proximity to China and the fact that Vietnam was the last Mandarin culture. Unfortunately, because it is so clearly laid out and so few foreigners speak Vietnamese, speakers tend to be unused to hearing mispronunciations. Furthermore, there is very little difference in tones to a non-native speaker (it gets better with time) so it's easy to make mistakes. Check out this video to see what I mean. That video isn't the best, but I think it conveys my point. I consider myself pretty good at dealing with new pronunciations, as well as tongue twisters, but Vietnamese tongue twisters are so, so hard.