Recommended to buy Vietnam Traffic Map book – cost around $US 15.00-20.00 in a bookstore (A4 size, hard cover, 12x pages):
Recommended to contact this personnel: http://www.gpsclub.vn/VN/GARMIN/
All my GPS devices are using maps from them. Great source, much more detail than Garmin official maps, and very responsive supports. They also provides GPS hardware for motorcycle as well.
To explore further, you can use South East Asia map by Open Street Map here: LINK
There are some travelers spent their time and knowledge to draw some very useful POI maps.
An example here with the POI map by traveler named Dao Kim Trang. The map looks like below image. Link to download full size of that map is HERE.
(In which all provinces, cities are listed; connection lines between cities are the road, with road name (number) as well as distance)
Besides, you can use online maps (i.e. Google Maps), they works quite well in Vietnam, with Vietnam 3G coverage. Vietnam 3G now is improved a lot, not expensive, but still lack of coverage for remote areas