Three weeks ago it was easy, they just took a photocopy of the passport's data page, so I guess that's registration. My fav seller (100k VND for a week) ran out of stock so I shelled out 160k at the next kiosk for the same.
Taking into account that my home operator's roaming in SE-Asia costs 12€/MB (no kidding) buying local SIMs is just a non-brainer.