There are 4 subspecies of the Light-vented Bulbul (Pycnonotus sinensis) of which 2 occur in Vietnam.
This is the subspecies P.s. hainanus which occurs in the north of Vietnam.

1st, 2nd & 3rd photo: Cat Ba Island, Vietnam, 5 January 2024
4th & 5th photo: Cat Ba National Park, Cat Ba Island, Vietnam, 7 January 2024