Good (and rare) post, David Bass. I would have expanded more in my earlier one, but England v. Wales took priority
My background in Terminal ATC spanned the decades from the old GCA truck MPN11 to AR1 and AR15. In Area Radar (more pertinent here) I spent years working the T82 (with stacked beam height-finder capability) and remotely fed Air Defence T84/T85. Then at LATCC with its multitudes of radars feeding a computerised combining machine that gave the best radar cover for each 15x15 mile box.
I suspect the Malaysian and Vietnamese authorities will have some radar recordings to assist, once processed. Whether either one wishes to reveal the extent of their Air Defence radar cover is a separate issue. I suspect there would be discreet hints to their National search assets, but I doubt we would see that revealed.