In november this year I would like go to Vietnam-Cambodia-Thailand
Do you think is dangerous there?
I would like to go there with rent or buy an old motorcycle.
Any suggestions?

It will be dangerous if you rent/buy a motorcycle. Motorcycle accidents are the leading cause of death for both locals and tourists in these countries.

What do you suggest then,rend a car?

No. Go from city to city via train, bus or flight.

How much time do you have? Do you have an itinerary yet? You will likely want to use different modes of transport over the course of you trip.

I have 2 weeks! I would like to use different modes of transport

It will be cool

In two weeks, you don't want to try to cover all three countries. Pause and consider what attracts you most to this part of the world, then plan your itinerary around that.

Riding motorcycles are dangerous depending on your skill level. I have been on motorcycles for 35 years. I have logged 25,000+ kilometers on my Yamaha 135CC Exciter. I do not hesitate to go anywhere. I have been in minor bumps and scrapes but have not been injured more than a bruise. I ride with my wife and 5 year old daughter. There are many places a somewhat unskilled rider may rent and ride, for example Phou Quoc is a great place for someone to familiarize themselves and have a Vietnam motorbiking adventure.

spottedgecko, do you live in VN? Even proficient riders can get into difficulty with VN traffic. And most westerners are accustomed to riding much more powerful bikes, so can mis-estimate the time they need to pass or whatever.