Your flight can actually be early morning on the 20th however you must go through immigration before 11:59 as stated above. Once you go through immigration you are considered not in Thailand ๐น๐ญ.
What ever you do, do not go back out into the airport as when you back through security/immigration you will be an overstay if it is after midnight.
there was four of us and I think the cost for everything the help, the van, driver and fuel was around 4,000 baht. Of course there are the visa costs 1,700 baht. Donโt bother taking dollars as all mine were rejected. They readily accepted Thai Baht everywhere including for the visa. For US the visa cost 40$; but they want baht, do the math.
Right before you cross over from Thailand ๐น๐ญ at the friendship bridge into Vientiane there is a tour company that will take from Thailand to Laos, help with all the paperwork clear through the Thai side and the Laos side. They will take you around to the sights, they know the good places to eat etc. our Laos visa took about 15-20 minutes total ๐
It is located just before this sign on the left hand side.