It will be very painful to get from Hoi An to Nha Trang as it is a 12h bus ride.
I was thinking to take a sleeper bus, which will be the cheaper alternative even if it is very long.
The other alternative will be to take the plane, however all will depend on the prices of the tickets and I don’t want to book that now as I want to get flexibility of travel.
However, I found some nice special deals with the company Vietnam Airlines where there are some flights from 333000 Dongs which is the equivalent of less than 15 euros. It can be a good alternative as I won’t stay that much time in the country.
I will stay at least 2 full days there and I might spend an extra night there to relax and enjoy a SPA before to get back travelling on the next morning.