Everyone else is always worried at the start. I am a first year teacher in China and I think everyone has these feelings at the start, but they go away once you become accustomed to your new job and lifestyle. The experience you'll have- I'm sure- will be a fantastic one.
Remember that you are flying from a native English speaking country to a country where there will be far less native speakers, so I am quite confident the school will be glad to have you there. Regarding your knowledge of grammar and verbs, you can always read more about this and 'brush up' on your weaknesses. Anyway, in my experience (of living in China anyway) foreign teachers tend to be given Oral English and Pronunciation classes to teach. Vietnamese students will not benefit from you teaching them grammar, this would be better covered by a Vietnamese teacher who can explain rules in their language far better.
Regarding lesson plans some teachers will have the same class all week, so can get away with one lesson plan per week. However I suggest you build resources now of: games you can play, topics that may interest students and materials you can take from your country (for example photos of where you live, the currency of your country, street maps, timetables-these will fascinate many students and can be used for classes).
What to expect at the start: what will you do if your teaching Oral English and no-one wants to talk in class? What will you do if a student starts to watch a personal movie in class, how will you discipline students? How are you going to assess students? How can you test students knowledge and ability when the classes are so big? Personally I find the language barrier a problem - sometimes students want to tell you something but cannot do so politely so maybe they'll say "you're wrong" or "your lessons are dull", at the start I found this very rude but now I am used to their feedback.
Language: what is the local language for "stand up, sit down, repeat after me, homework, next class, every class, test, exam" - you ll find this all on the internet.
Finally my advice to you is find out as much information as possible about your new job BEFORE you arrive there. What are the class sizes? What subjects will you teach? Which majors will you teach (English majors, non English majors) ? Then buy a couple of books before you go. Buy a good book (depending on what you will teach). Take a good dictionary with you (I brought a dictionary of about 3000 words (far too small), only to find out when I arrived that Chinese students need to learn about 8000 words for their exams at university). A dictionary will also show you the phonetic/pronunciation of words, grammer etc. REMEMBER it may be hard to find books once you're in the country.