nothing official, but I think they want to see income you can "live" in thailand with... who knows, they used (or maybe still do) to randomly ask for 20k thb from tourists coming in... maybe thats the amount?
You could have the vietnam ticket ready and explain the IO about your plans to go and get a proper visa. I would think that they understand, they can grant you shorter stamp too until your planned trip, if they dont want to give the full exempt