@Roland *****
IF you're going to cross into thailand by the land border at Nong Khai you won't be asked for anything, not proof of funds, proof of onward, travel proof of hotel booking, not nothing.
BUT seeing as you're going to be in Vientiane why not snap up a single entry tourist visa from the consulate there? That gets you stamped in for 60 days when you enter the country AND you wouldn't be burning one of the TWO 30 day visa exempt entries by land you're allowed in a calendar year either.. <- Just a thought ;)