It appears Facebook never formally "launched" a Vietnamese site. Facebook asked for users to help translate the site in June of 2008:
Based on this feedback, we opened 55 new languages this week for
translation by the community. Now native speakers can use our
Translations application to translate Facebook into additional Asian
languages (Malaysian, Vietnamese), African dialects (Zulu, Xhosa),
regional varieties (British English, Canadian French) and even rarely
spoken languages, such as Latin and Esperanto.
And the Vietname version grew out of that. This blog in October 2008 notes
I recently checked the translation functionality of Facebook for Tiếng Việt (Vietnamese), and it was, well, really cool. The wording is quite accurate, and fits with the context of Vietnam.
It was a surprise for me that Facebook in Vietnamese could go that fast without any fanfare.
So it appears within four months the site was translated to the point of being functional. But there was never any further announcement made, which is likely what made your search difficult.