Some of the stilts at Pulau Burung have extensive black on the back of the neck, and look somewhat similar to White-headed Stilts. There is a single sight record of this taxon from Cochinchina, so I have been wondering how to distinguish them with certainty from Black-winged. Here's a bird at Pulau Burung:
And here's a White-headed Stilt I photographed in Sydney, Australia:
From what I can see here, White-headed Stilt has a 'mane' at the back of the neck, which sticks out slightly. Also, the bill appears slightly upturned, especially the lower mandible, and looks sharper tipped. Finally, the primaries seem to extend slightly further beyond the tail on White-headed, though this is difficult to gauge.
It certainly seems that black nape markings are not unusual on Black-winged Stilts here.