Found a paper on-line (see link below) which extends the range of the mandarin ratsnake 970 km farther south into south-central Vietnam (Mount Ngoc Linh on Kon Tum plateau). Apparently the montane environment of this locale is cool enough to accommodate the mandarins even though the lower altitudes below are thoroughly tropical. It's like the montane species in the southwest existing on the summits of mountains only and therefore being separated into islands by the intervening warmer lowlands.

http://www.gli.cas.cz/seh/files/SEH_Abstracts_Final_20-09.pdf

(see page 85 of 135 under title "Herpetofauna of Central Highland: level of endemism and representation of Tonkin-Himalayan and South-Indomalayan elements"

I wonder if these new mandarins look like my viets? Wish there had been pictures.