Cambodia-Laos Don Khom Border
Given that there is only one border crossing, this must be the Don Khom (Veun Kham) border?
Sorry this might not help but back in 2009 we crossed this border and we already had our visas (so we would have 30 days).
It is a (VERY) small border so probably best to get a Laos visa beforehand (especially if you want to stay longer than 15 days) but from non conclusive post I read on GT Rider
they also issue on-the-spot entry visas. This is just a stamp and date/signature in your passport and is only valid 15 days.
NB we were asked by Cambodian official for US$5 to stamp our Carnet.
We loved Laos, enjoy and do go to Khong Lor caves, simply incredible.