You never travelled on a motorbike? You have never before been to Vietnam? No worries, I can give you some tips: easy riding is safe and very easy :-)
As weather is changing very quickly as well as climatic differences between the coast and the highland, it will be best, if you have clothes for all weather: make it "onion-style", so you can take off your pullover if it gets to hot or you can put on your wind breaker, if it is getting too cool while easy riding.
When the weather is called and rainy on the coast, you will love the warm sunshine of the central Highland. If the coast is very hot, up in the highland you will find Temperatures in the mid-twenties in the highland.
Put on enough sun protection as we drive through the sun all day and there is mostly no shade on the way :-) Dont't forget sunglasses to protect your eyes!
I will provide a very good raincoat for you. Just take some (open?) shoes with you, that can get wet and dry easily afterwards. For the sunny days, I suggest you wear trainers to be able to do some short walks.
There is some swimming-holes near waterfalls, in lakes or the ocean: bring your swim things in your day pack with you, that you can enjoy the easy-riding-breaks to cool off:-)
Apart from that, you can buy anything like drinks and food on the way. I organise accomodation which mostly have internet connection of some sort.
If you don't want to get all the exhaust of the cars and motorbikes driving in front, a facemask can be useful (You can buy them for very cheap anywhere in Vietnam).