If you are flying into Hanoi there are sometimes bikes for sale, the occasional Minsk if that is what you fancy, in Hanoi backpackers. My suggestion would be if you have limited time, two weeks or so, take an organised tour, if longer buy a bike and do your own.
If you have longer do go into Laos and Cambodia, Vietnam is a very busy place the others have more space and off the beaten tracks places. A Vietnamese registered bike can go to the others but not vise-versa.
If gaffer tape doesn't fix it then you haven't used enough tape