You need a visa invitation letter prior to arrival. It can be obtained online but shop around. I had been getting the letter for $25 but my friend in VN got it for me for only $8. I'd been getting ripped off all this time and didn't know it.
The last time I had a photo ready and they dismissively waved it off saying "no need!" Only thing I would caution is at the airport, give yourself at least 1.5 hrs for re-entry permit before your boarding time because now they make you wait in a separate queue next to the immigration queue for quite a while. I nearly missed my flight because it took them almost half an hour to get me the permit.