There are a few motorcycle rental shops in Vientiane. Douang Deuane Hotel, PVO and Lucky Motorcycle Rental (not sure if they are related to Lucky! Lucky! in Cambodia) are listed in the Lonely Planet Laos guide and PVO is recommended as the best place for 250cc bikes hire. A Honda Baja at PVO cost US$20 per day.
Another place that rents out scramblers is Green Discovery, a company that also conduct motorcycle tours. Depending on the number of days of hire, they charge between US$25 to US$28 per day for a Honda Baja. A useful service they provide is that they allow one to pick up or drop off the bike at any of their branch offices. The branch offices are located in Luang Namtha, Vang Vieng, Luang Prabang, Khammouane and Champasak while the head office is in Vientiane. It cost between US$20 to US$40 for the service.
I ended up choosing PVO as they are cheaper than Green Discovery. However, I would seriously consider Green Discovery if I am not traveling in a loop. They would be a good choice for a Southern Laos ride from Vientiane to Pakse (near Champasak).