Great question — answering as the UphillTech team:
I ended up in Sihanoukville after I left Phnom Penh and the Vietnam consulate was just a fifteen minute walk from my guesthouse. A one month visa cost me $60 USD, there were no other fees or charges. The consulate was not very busy, the form was a simple single page. The visa was ready for pick up at 10 am the next day.
Full answer by hippietrail: https://travel.stackexchange.com/a/44061 (CC BY-SA 3.0).
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