If you don't know how to choose English as the interface language, please
When you're in the forum, scroll down the page to the footer, there you'll see the selection below:
Click on it and select English from the drop-down menu
I'm sorry to say that VnVista no longer supports English as much as it used to for quite a while. A large number of new features have been added over the time, but there's been no translation for English-speaking users. It is due to my always heavy workload that I can find no time to do the translation.
That said, if you are a foreigner and interested in some VnVista feature, as much as I am happy to see that the site has reached non-Vietnamese users, I can't help you much with the language. The most basic controls and paginations have been translated, but not the newer ones (like those in the control panel for blog service, social networking, and other areas in there). If you want to use them, I'm afraid you'll have to switch to Vietnamese interface, and use a Vietnamese-English Dictionary to translate the guides for those features that do not have English translation. One very good online Dictionary I can recommend is vietdic (which has not only English-Vietnamese and Vietnamese-English dictionaries, but also Vietnamese and German, Frech Dictionary). So please go there and do the translation for yourself.
Anyway, in the future I'm sure VnVista will support both Vietnamese for Vietnamese users and English for foreigners. But for now, just can say sorry for the inconvenience.