We have 3 routes from Hanoi to HCMC and vice versa. These routes take 2 weeks but I would recommend taking 3 weeks to do them. The routes are largely the same but vary from Hoi An to Dalat. The routes have the option of seeing Mai Chau, Ninh Binh and / or Cat Ba island outside of Hanoi and the these options can be selected when the map is downloaded. After seeing these places, head south inland through Phong Nha and Khe Sanh then to the coast for Hue and Hoi An. They all then differ on the way to Dalat and follow the same path back to HCMC.
If you have a month or more for the trip, you should also select another route for the north of Vietnam from Hanoi. You can see the routes here.
This route stays inland from Hoi An to Dalat through the centre highlands. It offers more quiet roads with nice mountain views but without any of the stops or rides being special or standing out. If you aren’t looking for beaches and want to avoid the more touristy areas, this is a good option.
This route sticks to the coast from Hoi An to Nha Trang then comes inland to Dalat. Through this route you get some nice beach towns such as Quang Ngai and Quy Nhon which are untouristy and with stunning beaches as well as the largest tourist city of Nha Trang. If you want to see the beaches that Vietnam has to offer, this is best route for you but it comes at the expense of having a busier road to follow.
Mountains and the Beaches
This is the most suitable route for most people but takes at least 1 day more than the 2 above. It follows the same route as the 2 above. It also directs you to Pleiku from Quy Nhon which results in 1 extra day on QL19 . The benefit of this route is that you get to see the nicer beach of Quy Nhon and a more scenic route from Dalat to Nha Trang which is the highlights of the beaches route. So long as you can afford an extra day and are happy to see some beaches and some mountains, this is the best route.