Go to https://www.openstreetmap.org/#map=6/18.844/106.677 and zoom in and you’ll see the level of detail that there is, as rendered on a map.

If you want to download all of the data that OSM has about Vietnam you can do so from http://download.geofabrik.de/asia/vietnam.html - but that is unlikely to be useful to you unless you have something that you plan to do with raw OSM data.

What I suspect you’re more likely to want is data in a specific format that is understodd by a particular piece of software or hardware. For example, if you wanted data for use with a Garmin sat-nav then you could get it from http://garmin.openstreetmap.nl/ (among other places).