I just crossed into Thailand from Laos at the Vientiane / Nong Khai border. As I was after a 60 day visa for me and my truck I thought I'd share the process I went through.
Drive your vehicle as far to the front as possible then park and get out. Head to any of the offices with your departure slip and passport. Hand it in to one window (for immigration) and you should get it back from the next window. Then you got to the vehicle exit window and pay approx 50,000 kip as an exit fee.
You will then be allowed to exit. You have to pay another fee approx 8,000 kip for the toll to cross the Friendship bridge.
Note: these fees differ depending on your vehicle. I recommend to take 60-100,00 kip to get across. I managed to blag my way with only baht but they gave me kip back in exchange for baht. I probably lost out on the exchange.
Once across to need to follow the cars to the right handside and then park before the immigration offices. Though if there is no parking available you can probably drive through and park on the other side. There was nobody to inspect or stop the vehicles. The onus is on you to get your paperwork completed.
First stop is office 7a to pick up one forms (that they fill out), and an immigration card (you fill out). You need your vehicle ownership papers and passport.
Then go next door to 4a or 5a for immigration with your immigration card completed and pay 200baht fee. This is deals with your personal visa. If you have a visa for 60 days that you have picked up from the Thai consulate in Vientiane before crossing you will get 60 days stamp. Make sure they know its a 60 day visa.
Next stop is 6a for customs. Show them your vehicle registration papers. I also had my proof of purchase invoice but they didn't seem interest in that. Best to have anyway just in case. Here they will give you 30 days as standard. You need to ask explicitly for 60 days like you visa (if you are after 60 days, if not no worries). You will get passed about but you need to go into the customs office on the left hand side of the complex (as you look into Thailand) to see the officer here. Explain you need 60 days inline with your personal visa and they will ask you to fill out a request form.
They will require photocopies of passport and entry stamp along with vehicle ownership V5 and purchase receipt document. There is a photocopier at the entrance for 5 baht per page. The customs officer will then change the date to 60 days.
Then you head to 6d for final paperwork and your done. Easy!!
Nobody even checked my truck. The guarantee mentioned on the website is a figure they add to the customs document. No cash deposit is required.
Link to Thai customs website
I hope this helps. All up it took me about two hours to cross the border. With this info you'll probably save 30-45 mins wondering around trying to figure the process out.