Butterflies in the genus Mycalesis, the Bush Browns, are common in the warm regions from Central Asia to Australia.
Predominantly brown in colour, all the species of the Bush Browns have submarginal ocelli (eyespots) on the underside of both the fore and hindwings.
Their flight is generally weak but they are alert and skittish, and are often quite challenging to photograph. 
The Bush Browns typically rest with their wings folded upright.
They are notably polymorphic, with wet- and dry-season forms differing in many species, especially as regards size and number of underwing eyespots.
While the ocelli in the wet-season forms are usually well defined and clearly visible, the ocelli in the dry-season forms are usually faded and small.

The butterfly in the photo above looks very similar to Mycalesis mineus, Mycalesis mucianus and Mycalesis perseus (or perhaps another within this genus).
It is very difficult to distinguish these butterflies from each other without clearly seeing the upperside of the fore- and hindwings.
The photo only shows the underside.
Mycalesis Mucianus can normally be recognized by the shape of the forewing tip (not round but flattened), but it is damaged here so it is impossible to say.

Dry-season form. Cat Ba National Park, Cat Ba Island, Vietnam, 6 January 2024