Best route to take? I suggest you take the coastal roads along the ocean until you reach Dong Hoi, turn left to the Ho Chi Minh trail. With that route, you can see the ocean and the mountain (the Ho Chi Minh trail) and look out for blue signs on the Ho Chi Minh trail for many historical places. The Ho Chi Minh trail itself is already a "war scene" (read about it for more info). The mountains are higher in the North, more fun, so that's why I suggest you take it from there.
You will see many awesome back roads as well. I especially enjoy the ride from Long Hai beach to Mui Ne. It was empty, the road condition is really good, the sand dunes view was on my left hand side and the ocean view was on my right hand side.
It was just my suggestion! Have a great trip!
P/S: If you need help with buying bikes, let me know. I can help.