Unicode Problem in posting

[melodieskingdom]

Hi there, I've problem with Unicode here
I've set charset = UTF-8 in the language index file, and my site runs quite well with it, only a problem with posting:
With Unicode, the subject field of the post performs correctly, but the body text doesn't. I could'nt understand why, but I guess it would be the way SMF write down data to database, then different ways in writing down the subject and the body text produce different results.
For example:
Here the text I want to see on my site:

Chào mừng đến Diễn đàn! (oh it's Vietnamese, and it means "Welcome to my forum!")

If I type that text into the subject field, I get a good result:

Chào mừng đến Diễn đàn!

But with the body, it'll be:

Chænbsp;o mừng đến Diễn đænbsp;n

So could you please tell me the way SMF write down data to database in posting ?

Thank you in advance and sorry about my English.