I recently travelled to Vietnam and was looking at exchange rates for some leftover cash.

I noticed a huge swing last week (June 26) where the value of the VND basically doubled for a couple of hours (see the chart in the link). This change is reflected across multiple currencies, so it appears to have been a real event.

I looked at the 10 year chart and this is not a regular occurrence. Does anyone know what happened or how I would find out?