What are the best travel options from Udon Thani to Vientiane?

Your best off catching the Udon Thani-Vientiane cross-border bus, which drops you off in town. However, I think they leave from the bus station not the airport, whereas, there's always a van shuttle waiting at the airport entrance going directly for the border, which tends to be faster, even though you have to change transportation twice: first at the border, then you catch a local bus across the bridge and finally there's a change of transportation on the Lao side heading into the city.

You can also catch a train across the border as well, which may be faster in terms of immigration procedures, however, you'd need to show up close to the departure time of the train as there are only two services in each direction a day.

It’s cheaper to do it without nok air (have done with and without nok air). When you come out inside the airport, look for the desk with the sign that says ‘limousine service.’ You can book a seat in the mini van for 200 baht per person. Ask to go to the friendship bridge and they’ll drop you at the border. From the border into Vientiane, there’ll be people offering you a ride in. We got a van in for about 400 baht I think, there were 5 of us and we got the price down. But cost 700 baht arranged from our accomodation back to the border the next day. Hope that helps.