we are a visa run company and , already, since 2014, we have been advised that South Korean can not do a border run at Thai-Cambodian border or at Kanchanaburi. We have never been shown a notification, I think it has multiple reason which I not want to list as this is guesswork only. The message is either Fly OUT/IN or get a proper visa. Sorry.
running a visa service to Vientiane I can confirm that OP will be fine. Just passport, copy passport id page, 3pictures (3.5x4.5cm) are required. They will ask for copy of your Laos visa and entry stamp. Fill out the application correctly and all will be good.