Laos forum

VTE-Pak Lai boat:
http://www.travelfish.org/board/post/laos/12129_pak-lai-to-vientiane-by-boat
http://www.lonelyplanet.com/thorntree/thread.jspa?threadID=1973954
even so, i wonder how long passenger boats will continue to ply this route since the Pak Lai-Xanakham-VTE road has been improved & is now served by buses (including one that goes all the way to Sayabouly town). maybe check again closer to June 2011.

between Pak Lai & LPB:
hasn't been any regular passenger boat service for years (even back in 2006).
there are cargo boats, not sure how frequent (might have to wait for days before the next one sets off) & how easy/possible it is to negotiate for a space on board (do you speak Lao?).

cargo passenger boat
cargo boats & passenger boats (that take on a bit of smaller cargo) are quite different - see pics here:
http://laomeow.blogspot.com/2010/02/311206-pak-beng-cargo-boat-landing.html

Following advice of Lonely Planet guide (and some research on internet) I was in Paklai today (Monday) hoping to find a regular boat for Vientiane. Sadly the boat no longer exists - I cannot assert with certitude it is definitely over, it might be a seasonal problem, but it seems likely this boat is dead.

Note that I took the bus ride instead, and that it is quite nice (forested hills, beautiful stretches along the Mekong) though a bit tiring (half the route is on unsealed roads).