Vietnam forum

So you've got around ten weeks?

A lot would depend on which route from Laos to Vietnam you take. If you take the northern route (through Phongsali and Dien Bien Phu) that lands you well for northern Vietnam, but will mean you'd need to allow a solid month in Vietnam if you're planning on travelling the full length of it before exiting into Cambodia.

If you use a more central crossing (entering in for Hue or Vinh) you'll be doing some backtracking heading north then south etc.

What I'd suggest it take a look at some of the suggested itineraries to give you an idea of how long it takes to get around.
http://www.travelfish.org/trip_planner/thailand-indochina-tour1

The above trip takes you to Laos and Cambodia before Vietnam -- this reduces the amount of backtracking.

Vietnam will be the main time-sink, Laos it depends on if you're seeing the north and south or just one or the other.

The above are just some thought off the cuff -- there's many many ways to skin this particular cat -- if you come back with an idea of a few of the places you want to visit, I'll be better able to suggest a routing.

Cheers!