Someone will probably come in and say it's the Ớt hiểm. That person would be semi right.
That is the vietnamese name for bird's eye type chilis and literally translates to something like "pepper that can't be trusted", referring to the mean and sharp type of pungency these small chilis have. They don't necessarily have to be very strong on scovilles though. So yep.. It's Ớt hiểm but it probably also has a local name, which tends to differ in all regions for the same pepper.