A historian speaks: this path was not one, but several routes through Vietnam, Laos and Cambodia. A bit like the silk route, it describes a number of ways from A to B.
There are practical difficulties nowdays in going in and out of Vietnam with a bike above 175cc. Similarly, not all the border crossing between these countries are open to foreigners.
You thinking of doing it? I am sure some version is possible.
Have a look around the BB and people's stories and you will get an idea of what is and isn't doable.
It seems an excellent theme for a bike journey.
Around the world 2000-2004, on a 1993 Honda Transalp