utf font display correctly in Mozzilla, but squares in IE

Sounds like a problem with IE missing the necessary fonts to display the characters you’re using. The only thing I can suggest is to specify a font in the CSS that contains those characters, like Arial Unicode MS or Lucienda Sans Unicode. IE6 is not so good at automatically picking the right fonts to use when it needs to display unicode chars. Even looking at your site in Firefox, the problem characters are rendered in a different font there. It’s close, but still decidedly different.

I thought it was only problem with IE in my computer. I’ve tried my sis computer, and even computers in my school. If it is required to install the right font on every new computer then it’s more problem of the script, is it?

I don’t thik specify the font would help, but I’ll try, any other suggestion? Thanks for the help.

Some fonts (font sets) simply don’t have all the characters for other than the basic English (ascii).

Fonts are theme-specific, so it is not really a “script problem”, more a theme problem. You could try:
a) changing the theme and see if it works
b) changing the base font for the theme to one that you know for 100% it has Vietnamese characters – and check if it works

changed theme showed correct font in IE, so this is theme problem. Thanks for all the help.