I live in Bangkok and own my KLX.. So no rentals.. But if you do want a rental bike in Laos.. Check out Fuark. Really cool Lao guy.. He is the main man most of the rest rent from and then rent on to you all ;-)
Not much English is spoken.. but in person his english is fine..
Based in Vientiane.. If you look on www.gt-rider.com
you can find Fuark and other bike rental details.