In fact I booked a bus to Cambodia and used that as evidence as leaving the country when applying for my 60 day single entry tourist visa at the Thai embassy in KL. Cost $20.
Tod Daniels I arrive in Udon Thani at 4pm and plan to hand in my passport the next morning at Vientiane embassy. Will find a bus from the airport to Vientiane. Will collect the passport the following afternoon and take the 10pm flight back from Udon Thani to Bangkok. Should have plenty of time! Thanks!!