I've suggested this get moved to the Touring subforum.
https://en.wikipedia.org/wiki/Geography_of_Vietnam
So, in the middle, Vietnam is only 50km wide. You could easily do that trek in a day, assuming good roads.
It is a little wider at the two ends, and about 1650km (about 1000 miles) long, total.
A lot will depend on your actual road conditions. Paved? Gravel? Dirt? Course ballast?
At this point, start planning your route (bike paths, and secondary roads). Find a good bike that you would be comfortable riding on for quite some time.
Find your gear, or choose to go "credit card touring".