LO,
Perhaps I can help. VTO has been a requirement for F-35 since Day One.
However, as the guys writing the requirements were technically informed, they didn't try to make the aircraft do a VTO with an operational load, as they knew that this was beyond the laws of physics as they stood in 2000. However, they did want the ability to use VTO to reposition an F-35 from one deck to another. The 'Atlantic Conveyor' was a very pertinent example - and the US War Colleges have been using the Falklands War as a textbook example of tactical flexibility these past 30 odd years.
Re the speed limits for transitions - thanks to SpazSinbad for a really useful post. Thanks, wings.
Hope this helps
Engines