It might be the female of Derolus blaisei Pic, 1923. Plavilstshikov (1931) provided the descriptions of all Vietnamese Pic's Derolus-species, polemically criticising and synonymising them. Actually, Gressitt & Rondon (1970) provided the pictures of several Pic's species, confirming their validity. It is not easy to identify these species using those short Latin descriptions, but it seems to me that D. blaisei might fit to your specimen.