All this information does take us much farther than what is found anywhere else.
Now we have a snapshot of the one minute and forty six seconds of time when it appears that control was lost.
The last FR24 data point was: 23:12:37 3.990 s 110.225 e, which is at (about) the same time that Lost in Saigon informs us:
06:12
- QZ8501 requests left deviation from airway. Deviation approved.
- Pilot then requests climb to FL380
- ATC asks pilot to standby, due to nearby traffic and to coordinate with next sector (Singapore)
Thus we can assume that the plane had converted its forward speed to vertical speed at or near the time the pilot was requesting FL380.