Vientiane is exactly as they post it (well sort of, they count days squirelly though) You make an appointment show up on that day, time, turn in your application pay the 2000baht, show up the next working day after 1PM and get your passport back with the visa in it (they call it two days after submitting but it's the next day because the day you submit your paperwork is day ONE, π )
you definitely need to book the appointment for Vientiane or you won't get in the gate.
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Phnom Penh is exactly as they post it, show up, turn in your application pay the 80USD and then TWO working days later show back up to get the passport back with the visa in it.
There are reports from people who have used Phnom Penh that they were able to get the passport back the next day BUT then other people reported it went back to 2 days later like they state SO that one is a "your mileage may vary" sort of consulate to deal with.
you need to read the requirements that each consulate lists as far as what documentation you need to get the 90 day single entry Non-B issued because they are not the same between the two consulates in Vientiane and Phnom Penh π