Hi,

Borders:
it's definitely the best way to go on Vietnamese plates. In 9 out of 10 cases your papers will be extremely dodgy, but don't worry, still works just fine. You can't get your name in there but that again is no problem. Officially Thailand and Malaysia want to have a signed copy of the "real" owner, in my case I didn't need any for thailand and Malaysia is a drive-thru Border, not stepping of the bike and 15 seconds brings you in.
Singapur also should be no problem on a vietnamese bike, cause as far as I understand it, all the countries here are members of the ASEAN or so, meaning they accept the bikes of the other countries (in theory). But if they say no I wouldn't bother too long cause it's not worth the hassle. And for the other borders, if you really should ever get into trouble, just give a local 5$ and he'll bring it over for you as they don't even need papers, making your exit of the country easier as well. But with the Waves/Dreams they never look at them at the borders, ever.

Bike:
Honda Wave/Dream is the easiest and safest bet. As said, parts, mechanics and labour throuout SEA. If I were you I would take a Wave as the suspension on the back is stronger ( if original, rare). If you are tall, say over 1,85m it might be smart to get a fully automatic one cause to shift down on the step-through is annoying to impossible for tall people. Before leaving Vietnam, buy parts you think will die soon or let someone fix it, as prices for parts/labour for the countries goes like this: Vietnam<Laos<Cambodia<Thailand<<<Malaysia. Also I'd recommend getting extra seat padding customized in HCM (4-15$, 15 includes cool leather or hello Kitty pattern) to do your ass some good as the sitting positing and the low bike make for some serious pain or you can try, suffer and then do it in Hanoi where it's cheaper.

Time
is up to you, in theory you can do Hanoi to HCMC in 8 days, in reality it took me 2 months and regretting to not having stayed longer in the north.
I'm now in KL, startet in Hanoi on, the untrustable but more romatic, minsk. It took me 6 months with around 3 weeks repairing, hehe. But I took it pretty easy and I did around 13.000 km.

Cost
is low. How I said, Vietnam is by far the cheapest, you can get double air-con rooms in the country for as low as 60.000 Dong (3 Dollars) and usually they let a third person in, sometimes charge a little extra, like 10k. Food is dirtcheap in all countries. My daily average is 17,60 Euro right now, including a newly bought SLR-Camera after mine was stolen so that's 1000$ not really counting. And as you are 3 people, I'm on my own, you can ususally safe money with 3-people rooms.

if I may a tip:
the cities for Vietnam you listed sound like you're thinking of taking Highway 1. Dont. Period.
It is always full and crowded and all the trucks and buses use that route, also the quality of the road mostly sucks . The HCMH (Ho Chi Mihn Highway) runs from HMC all the way to Haiphong (through Hanoi) through the by far better countryside with the not-soo-spoiled locals and cheaper prices. If you care I can recommend you some real nice roads, just drop me a line or write it in here as others might care as well.


Enjoy you're trip, and don't think too much before coming, things get easier as soon as you're here.

Cheers
Tim