Yellow-vented bird

The yellow-vented birds in Vietnam could be:

1/ Yellow-vented green pigeon (Treron seimundi), 
also known as Seimunds's Pintail Pigeon

2/ Yellow-vented bulbul (Pycnonotus goiavier), 
or eastern yellow-vented bulbul,

3/ Yellow-vented warbler (Phylloscopus cantator) 

4/ Yellow-vented flowerpecker (Pachyglossa chrysorrhea)

5/ Sooty-headed bulbul (Pycnonotus aurigaster)
Only some individuals have a yellow vent, some have red vent.

6/ Brown-breasted bulbul (Pycnonotus xanthorrhous)