
These are species with at least one recorded territory in Vietnam, not a national checklist: a bird list from eBird or Avibase is more complete. The counts follow recording effort as much as wildlife, so a country with more visitors and more naturalists will show more of both.
The species with the most open occurrence records in Vietnam, among those a photograph exists for. Most recorded is not most numerous: it rewards animals that are common, conspicuous and live where people go.

Dicrurus leucophaeus

Aerodramus fuciphagus

Phylloscopus inornatus

Ardeola bacchus

Pycnonotus aurigaster

Pycnonotus finlaysoni

Pycnonotus conradi

Rubigula flaviventris

Orthotomus atrogularis

Hypsipetes leucocephalus

Rhipidura albicollis

Crypsirina temia
The reference database has no regions filed under Vietnam.
The field guide covers Vietnam on the map, but none of its species have an entry written for them yet.