What documents and requirements do I need to travel to Laos from Thailand on an O visa?

Fellow members, I am on an O visa based on marriage 12 month ME, my 90 days expires Dec 25th (so no chance of forgetting) I currently live in Hua Hin and planned to go on holiday to Vietnam for a week in December which satisfies the immigration law. After a month or more of discussion on the trip, I now learn that my wife has a University reunion on December 29th in Kalasin, (Thai people are not so good at conveying plans, even after 10 years of marriage in U.K.) so I now think it easier if I go to Vientiane for a holiday instead of Vietnam, I am planning on taking the car across the friendship bridge, what documents will I need, will I need a tourist visa to Laos and what issues am I to encounter ?

Not forgetting 20,000b evidence on my return.

TIA