What are the requirements for obtaining a 60-day Thai visa while in Vietnam?

Just did it this week. You'll need 6 months of bank statements with $600USD in the account. The website states 20,000thb, I had 20,000THB in the account, they said it wasn't enough and I should have more. You need flight itineraries for your flights both in and out of Thailand (actual flights, not one of those onward ticket "confirmations"), proof of accommodation reservation for your whole time in the country (obviously some bookings let you cancel for free and that's fine if you just need something to keep them happy, but beware that if you haven't yet paid for the accom yet, they may ask for more in your account, especially if the accom cost isn't covered by the amount in your bank account), you need 1 passport photo and a photocopy of your passport page. There's a copy shop next to the Mercure hotel nearby. It costs $40USD, but you need to pay in USD as they won't accept Baht or Dong. They've got the form you need there. I turned up an hour before opening (no booking required) and it was unnecessary, 10 mins before should be fine. The earlier the better, because most people aren't aware of everything they need and you'll end up stuck behind people who are unprepared. Or people who are prepared but get asked to jump through hoops they didn't know about. It can take 1-3 working days, mine took 2. Also, double check on Thai holidays as I rocked up on Friday and it was King Bhumibol Memorial Day so they were closed. My main advice is give yourself enough time.