Peter:
It's hard to say what the actual range is. But if the mandarins are like some other montane species then when they reach the southern (warmer) edge of their range they tend to be restricted to cooler mountain "islands". A topographic map may be a good way to find other locales for the southeast asian side of the mandarin range. Schulz suggests Laos as a possible range extension. Based on the physical differences between viet mandarins and chinese specimens I'd love to see further locales for comparison.
Thanks for the kind words on my poor attempts at photography. One day I'll get a better camera and more practice.

Best regards
John