I was told the 6 month was only on entry to Thailand. Can stay until 1 day remaining as long as your leave before expired date. But yes not easy to get a definitive answer.
I had similar 2 years ago my passport was in its expiry year. I wanted to fly Heathrow to bkk. I spoke to 2 different thai consulate staff. 1 said has to be over 6 months remaining for any kind of entry. The other said 30 day visa exempt not a issue. I never took the risk I flew with 6 months and 1 day remaining to be safe. I never really got a definitive answer to what is the correct ruling.
Vientiane does not ask for anything except 2 passport photos and your money. No onward travel or address or funds. But you may be asked when entering Thailand via border or airport.