Are you saying that you created a separate resource bundle with the Vietnamese language? Do you now have the two resource bundles? If so, you need to make sure that the bundles are named correctly. Typically this is something like resources_fr_CA for french and Canada, resources_ja_JP for japanese. I'm not sure for what vietnam would be exactly. Anyhow,
you should change the locale. There are quite a few ways to approach this. Do a search around to find how you want to accomplish this for your users needs.