This is the big one in Vietnam.  This is the test.  
I get that.  As a test it is important.  But it is a test, it is not English.
If don’t do very well on the IELTS, that doesn’t mean you can’t do well in English.
If you want to prepare for the IELTS I, I suggest a practice book, an IELTS preparation class, or this website  http://ielts-simon.com/ .
I like the material he puts up.  He updates regularly. He scores IELTS, so he knows what you need to do to do well on the IELTS.  You will learn how to take the IELTS test.
You will not be learning English.  Not really.  English is for people not tests.

Whatever you do, do it in English.  Most of the Vietnamese websites I see are in Vietnamese which……
If you can’t prepare for the IELTS in English, you can’t pass the IELTS.
Whatever you do, do it in English.

Good luck!  (And hard work 😉 )