Air Asia are now flying daily between Chiang Mai and Udon Thani for about 450 baht each way on most flights, at the moment. Useful if you're connecting to Vientiane for your visa of course.
TLDR : Answer Summary
Air Asia is currently offering daily flights between Chiang Mai and Udon Thani for approximately 450 baht each way, which is particularly useful for those connecting to Vientiane to manage their visa requirements.