Used a bus ticket myself from nong khai to Vientiane £12 was my proof of onward travel think it was 12go I used just Google bus nong khai Vientiane comes up it works as proof
I used an agent guaranteed visa or no payment got the visa no problem the agent was hanging around outside of embassy when approached us is a place in Pattaya that does same
I crossed over got stamped out of Thailand stamped in Laos stayed 3 days same coming back never went to no immigration to register I think you might be meaning crossing over then returning same day then you go immigration