What address should I use on my e-visa application if I am currently in Vietnam, and which immigration checkpoint should I choose when arriving in Thailand from Vientiane?

You are required to provide documentation confirming that your current location is Vietnam when submitting an application with a foreign passport. Acceptable documentation may include an entry stamp, a boarding pass, or written confirmation of your accommodation. The First Friendship Bridge serves as the border crossing between Vientiane and Udon Thani; however, you must document or specify the mode of transportation you intend to use for entry, but there’s no direct train for this route. Personally I would have chosen a direct bus all the way.