In the Ninh Binh area in North Vietnam three very similar looking Leaf Warblers occur in winter that have two distinct wingbars: Claudia’s Leaf-Warbler (Phylloscopus claudiae), Davison's Leaf Warbler (Phylloscopus intensior) and Kloss's Leaf-Warbler (Phylloscopus ogilviegranti).
I don't think it is Claudia’s Leaf Warbler, because the head and underparts are not cold greyish with a slight yellowish wash on the face and supercilium. 
Claudia’s Leaf Warbler also usually shows a contrast between the more greyish head and the olive back.
In addition, Claudia’s Leaf Warbler has a very unique way of foraging. 
It almost always clings on tree trunks and branches similar to a nuthatch.
The bird on the photo did't show this behaviour.
Compared to Davison's Leaf-Warbler, Kloss's Leaf-Warbler is usually brighter green above with even brighter yellow on the head and face.
The two species are barely distinguishable from each other and it is therefore not surprising that they used to be considered one and the same species.
However, in the Ninh Binh area the subspecies 'disturbans' of the Kloss's Leaf-Warbler occurs (in winter) which is less yellow on the head, face and underparts.
Also, Davison's Leaf Warbler has more white in the outer tail feathers, a brighter yellow supercilium and crown-stripe and more yellowish-tinged underparts than the subspecies 'disturbans' of the Kloss's Leaf-Warbler.
Based on the rather pale, not-yellowish appearance of the bird on my photos, I think my bird is a Kloss's Leaf-Warbler.

Tam Coc, Ninh Binh, Vietnam, 11 January 2024