I recommend you don't change the globalization settings, if you do that you will get errors when there are missing keys in the Vietnamese resource files.
By default it will automatically adjust to the user's browser language preference.
See the notes at the bottom of this page about how to force a specific culture http://www.mojoportal.com/localization.aspx
Bascially just add a setting in the appSettings section of web.config or user.config like
<add key="site1culture" value="ru-RU" /> would force to Russian.
However in all cases if the forced language is missing resource keys it willfall back to English which is the only one guaranteed to not have any missing keys, this is why you should not change the globalization element.
If you translate the .resx files and any files you tranlsate in the /Data/MessageTemplates folder or /Data/Help folder, please send them tome so I can include them in the project joe dot audette at g mail dotcom.