If driving, Avis seems to have the best deal with a Mahindra Skorpio (looks like a 4x4 4 dr p/u) as the lowest priced-option at LAK 7mm for a week. Avis is the only one offering the o/w rental. Laos Air has very good b/c deals if purchased sufficiently far in advance, e.g., I paid ~USD 110 (came through as less because I noticed the LAK price was lower than the USD price) for VTE-PKZ.

(manin)seat61 would have decent info on the train.