Is it possible to rent a bike in one city and drop off in another, when traveling to Vietnam?
It depends, namely on what cities you want to rent and drop off your bike in.
There are a few one-way rental companies with motorcycles targeted at tourists. The one I saw advertised most often when I was in Vietnam last month was Motorvina. These companies will let you do a one-way rental to and from particular cities, generally those that are major tourist draws.
The towns you've mentioned don't strike me as being particularly large or otherwise highly tourists, so it's quite unlikely that you'll be able to arrange a one-way rental between them.
Most tourists will instead purchase a motorcycle for use while visiting. These bikes are very affordable, and you will often find tourists in Hanoi or HCMC advertising their (sometimes modified for luggage) bikes for sale.