your best option is to have that in US dollars if you can. the last time i went through this process (in july) the lao officials who take the money were only authorized to take US dollars or thai baht, and the thai baht was accepted at a really unfavorable exchange rate. the guys in front of me had only sterling to pay for their visas, and after a long discussion, the lao officials agreed to take the notes, but at a pretty awful exchange rate.
i hope that helps. cheers.