Ubuntu font cannot display latin extended characters
I'm not sure about Vietnamese, but for example in Finnish, there are some extended characters that are problematic. This is because some pages are encoded as ISO-8859-15 and some are UTF-8. If autodetection in the browser fails, they will be displayed as boxes or garbage. The same thing happens if the font only supports one encoding and the page uses the other. I'd recommend to check the encoding of the font and the encoding of the pages you tried, and change the font if necessary.