I made this journey on 24 Oct. I booked an overnight sleeper ticket 2nd class air-con for the 2000 departure from BKK 36 hours prior to departure at Hua Lamphong Station. Cost 768 baht.
Train left on time - train staff made up the berth (lower one if you are not too agile) - request this when booking. Restaurant car available - snacks and drinks (including beer). A lady comes through the train to take orders for breakfast (if you so wish). It can be delivered to your berth. Train arrived Nong Khai approx. 30 minutes late (no problem). You can then transfer to a shuttle train (17baht !!) for the short journey across the Friendship Bridge to Laos and immigration. Cost 1500baht (UK passport holder). Queue up with a completed form (supplied at the station) - they take your form and passport - you move to the next window and you get your PP back duly stamped. When I went through there were no tuk-tuks or buses around. Only a couple of mini-vans. Two of us were charged 300baht each to Vientiane Centre.
Hope this might be of interest. If all goes according to plan it is a painless, and enjoyable, experience.