Not doing very well here. The only VN "Big" refs I can find are to "Big Mountain at Vung Tau" and "Big Minh."
There was a series of remote (cable) controlled crash-worthiness testing of old H-21s - on Utube
The last one could have ended up in the position of the challenge and with a similar shaped hill behind, but it claimed elsewhere it was done at Edwards Air Force Base, which to the best of my knowledge is in the USA. I assume we are still talking Vietnam for this challenge ?
None of these is offered as a serious answer. LFH