I applied online. I interviewed at Mekong Economics (Hanoi)
Interview
First they will give you a take home exam of SAT maths and two essay questions. once your writing skills are ok for them, you'll do an interview. I highly recommend spending more time on the essay because that's what they ultimately look for.
Interview questions [1]
Question 1
They'll ask you to write about a project in a developing country and you can google some resources to cite in your essay.