I think I may have stumbled on to a TAR sighting on Twitter. I searched on "amazing race" and after wading through about 10 million posts about TAR 14 which apparently just started running overseas (Australian TV?), I found this posted by the user girlclumsy (who appears to be an Aussie vacationing in Vietnam):
"Saw a finishing leg of "The Amazing Race" being filmed outside HCMC's Reunification Palace this morning."
With a follow up after a question:
"They actually pulled up in a cab, yelled "Go! Go!" like they were GIs, then ran (once cued) towards the host near a tank."
My co-worker, who has vacationed in Vietnam several times, verifies that there is a Reunification Palace in Ho Chi Minh City. He wasn't much on details though.