Visa required before going to Vietnam, can do online. Phenomenal Penh Cambodia has visa on arrival, they prefer US Dollars, cost may vary based on country, check online.
Koh Kong is worth spending a night or two, 400baht each way from the border, Cambodia frowns on baht, bring $USD. If trying an in-out you might have to quench the thirst of the personnel. On your return, there are some great beaches below Trat.
please describe the virtual mailbox. I just tried to change my address for SS and using a PO Box was rejected. I’m tempted to use the physical address of the PO but I’m hesitate at this point. My bank had no issues with the PO Box address.
If you’re flying to Phnom Penh you can fill out the form at immigration, it’s done on a tablet, $30USD fee, be sure to have that amount in cash, Thai baht is not accepted.