Hi, I just did a trip the other way around and can share a little experience.

Renting and scooters are great ways to go, the convenience cannot be understated especially if you are on a strict timeline. Buying is way more fun though.

I did get stuck trying to sell the bike in Hanoi because it's really a crap shoot whether or not you can find a buyer.

Anyways, I paid $350 for a Honda Win from a guy around the Pham Ngu Lao backpacker area. Danh Tranh, guy is not to be trusted. Told me he had connections I could sell the bike to in Hanoi, I got there and they complained he kept sending suckers up with that story and wouldn't give me half the 300 he'd told me. I ended up selling it for $140, but honestly it was worth it either way. A ripoff in Vietnam is a steal in the West.

The win is the "vietnamese light truck" as Ben from Phong Nha farmstay put it. The locals use it for everything, so it's ideal even though the cheaply made chinese honda clone _will_ rattle some bolts loose or what have you. Replacement parts are to be had everywhere, I got a new gearbox for $20. Ben also let me know $350 is the cost of a new bike, mine was at least ten years old and very few original parts from it made it all the way to Hanoi.
If I were to do it again, I'd have tried to make a connection with a local and have him buy the bike for me.

A word on the minsks, they're a popular dream but the number of them that are actually up to making the journey dwindles every month as tourists take them on yet another round through the country. Parts are way harder to come by, but if you get stuck on it at least there's a xe may (vehicle mechanic) within pushing distance almost everywhere, in the worst case.

You really want to do the HCM trail. Google Phong Nha Farmstay, it's not super budget but he's a really cool guy and they're nice digs. He's a font of knowledge. In the North make sure you ride to Sapa, it's a gorgous voyage. Everyone will tell you to stay off Highway 1, it really is a hell of a lot of traffic with the tourist buses running you off the road etc. but it's efficient as hell and not as crazy as they make it sound if you ride safe.

Hoi An is touristy as hell but still gorgeous, if you can time your visit to coincide with the full moon it's a wonderful experience. If you're going to Angkor Wat you'll be underwhelmed by the ruins of My Son in comparison but they're still cool.

Buy a Vietnamese phrasebook, you can get them for 2-5 bucks and it's amazing how the locals open up to you when you pull it out.

Phu Cuoc in the SouthWest is worth checking out, the deserted beaches to the North are lovely though not really deserted. It's nice to have that kind of space after the crowded mainland though.

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Lost on the way to the End of the World - Adventures of a vagabond photographer and his tRusty steed