There are 9 subspecies of the Sooty-headed Bulbul (Pycnonotus aurigaster) of which 2 occur in northern Vietnam, 1 in central Vietnam and 1 in southern Vietnam.
The 1-2 photos show the subspecies P.a. resurrectus with a red vent.
The 3-5 photos show the subspecies P.a. aurigaster with a yellow vent.

1st & 2nd photo: Lying Dragon Mountain, Tam Coc, Ninh Binh, Vietnam, 9 January 2024
3rd, 4th & 5th photo: My Son Sanctuary, Hoi An area, Vietnam, 30 December 2023