Yes it's true, a guide is not required. It was just a deal with my wife, she was ok for this trip if I didn't ride alone. I was lucky, because this young guy was very friendly, thanks to him I can share diners with locals and see the real vietnamian life.
I'm used to travel without a guide (see my previous reports), it was the first time for me so I had decided if the guide was not a good guy I continued alone, It has not been the case.
I was not really a guide who explain you history culture etc...more a trip companion. It was a good experience.
It is very easy to travel in Viet Nam without a guide.
The price for a 250cc is between 40 and 50$. Regarding the trafic you must be confident with a bike because you have thousands thing to take care.