I did this trip in '95 so my information will I'm sure, be somewhat out of date. Rather than renting, we bought 2 Russian Minsks from fellow travellers ending their trip in Siagon, having ridden down from Hanoi. One big advantage was the wealth of fresh info we recieved from them, having just ridden in the opposite direction. I don't know about now, but then there were quite a number of bikes being advertised in the back-packers cafe's. Minsks are (or were) to Vietnam what Enfields are to India, i.e. lots of spares and workshops throughout the country. Upon reaching Hanoi, we had both bikes sold within a week and at a small profit to boot!
Lots of good places to see, Hoi An I remember being especially interesting. We stuck to Highway 1, however when away from the tourist towns we were often asked if we were Russian (Hey Ruskie!), as the last Westeners they'd met were Russians who were present in the late '70's. Lot's of deserted beaches and bays to stop off for a quiet swim along this route too.
Have a good one, Hamish