>”How could the plane have then been on the radar 100 miles away in Indonesia if it crashed off the Vietnamese coast”<
The Radar signature you are referring to wasn’t the Plane’s Transponder signal.
The other Radar signature could have been anything.
“The Radar signature you are referring to wasnt the Planes Transponder signal”
It only takes the turn of one knob to change the transponder signal.
ATC issues the code that you put into the transponder.
You can squak 7500 for hijacking or 7600 for lost communications, all it takes is twisting of the 4 knobs to set any code you want.