We've done most of this on a Minsk 125cc (Russian) roadbike, two-up, so I reckon you'll be sweet, though my guess is you'll have a tough time keeping those mags shiny! (there's a rua xe (bikewash) every 2 metres in Vietnam, and they do like their bikes shiny). As for roads, Cambodia and Vietnam have many similarities when it comes to other road users, they're all pretty mad, but Cambodia's roads, although improving, are generally worse. Road 1 from Saigon to Hanoi has it's pothole moments, but the inland road from SGN to Dalat is good, and would be a good ride.Sorry, can't give you info about the Moc Bai border with a bike, but if you get there on sundown and don't want to do the run to SGN in the dark there is a guesthouse in a cross-roads town about 5-10mins from the border.
We did a circuit around Nthrn Vietnam, see that section for my info on that. The main roads in Laos are fantastic by anyone's standards, but there are exceptions. Beware of the clay roads after rain (very slippery), and the sharp potholes on once-were-well-sealed roads, and while we're at it, the culvert-like sudden dips on an otherwise flat roads. DON'T, under any circumstances, try and get you AND your bike through the Nam Can border (near Ky Son, Vietnam). On a map it looks like the ultimate border crossing for a loop around the north of Laos, before going south, but they will absolutely refuse to let the bike through, I don't think a Carnet will help anything. We tried and failed, Two days back down to the Cau Treo border, where they are pretty relaxed.The border between Laos and Cambodia is on a pretty bad road at Dong Kralaw (go straight ahead at the end of the good road onto the dirt track, don't go right to Voen Kham, unless you want to pay an exhorbitant amount to put your bike on a boat to Stung Treng). The border posts are very relaxed, charge $3 per passport on both Laos and Camb. side (I'm sure this isn't a set price but I wouldn't to hassle anyone when taking a bike through on foreign plates). The road between Stung Treng and Kratie IS THE WORST ROAD IN THE WORLD.
Hope this helps.
Bat