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Polygonia c-album
The Comma has a white marking on its underwings resembling a comma. The wings have a distinctive ragged edge, apparently a cryptic form as the butterfly resembles a fallen leaf.
tropical forest on limestone
I spotted this in Ba Be Lakes National Park. It was very difficult to photograph. I cropped it and zoomed in for a better view. It was on a steep hillside and I was leaning way over to photograph it. Looks like it's on some sort of Ageratum.
Gretel, you are right. It is a sub-species of the Comma, which is a widely spread species across the globe with a range that includes America, Europe & Asia!
Dan, I'm not sure exactly which subspecies this is but they are all listed in the wiki link so you can check out which one is geographically most likely to be found in Vietnam.