I am a Filipino citizen and will be conducting internal audit in Tay Ninh. I will be crossing the border from Phnom Penh, Cambodia. What are the things I should do or prepare?
As far as I can tell, the official answer seems to be nothing. As a Philippines passport holder, you don't need a visa for either country for stays of less than 21 days.
But since the ASEAN visa-free zone isn't quite as smooth in practice as it is in theory, do bring along any written documentation about why you are going to Vietnam, who you are meeting, and the phone numbers of any local Vietnamese staff who can smooth out any potential issues.