Facebook being blocked in vietnam?
If you imagine DNS servers as a phone book, then this analogy works.
The government in the past basically just removed the entries for facebook from the "phone book" so that computers couldn't "call" on facebook anymore. It is a very simple way to block something but easily circumvented if you change which phonebook you use (changing DNS server).
If, however, they now decide to go one step further which is to say, website traffic originating withing vietnam to the actual servers of facebook is blocked then using another DNS server will not work as they are effectively blocking the destination.
If this is what they are doing (my research seems to say so, but that they are testing it at the moment so that it works then stops then works again) then you will need some kind of Proxy server to route traffic through.
Hope the explanation helps people understand what had happened in the past and what will most likely happen in the future.
If there is interest, I could throw up a proxy server somewhere in the free world.