There is not a complete lack of radar data. There is some of the data available which can be found if you search for it - printed in Chinese media, as shown to Chinese relatives. However, not all the data is available as it seems the sensitive data of exactly how the turn around was achieved hasn't been seen by anyone in the public arena.
Vietnamese authorities advised Malaysia in real time of the westbound course but did not receive an official reply.
There was not an immediate connection between MH 370 dropping off communication and the westbound unidentified aircraft. It would be most natural to believe in the immediate aftermath that the aircraft was lost shortly after last contact and was in the South China Sea. The Malaysian Prime Minister did not want in the circumstances to make a definitive public announcement of a connection of the lost aircraft and the westbound track until other data corroborated the primary radar data. Whether this was wise in hindsight is another matter but it is unusual behaviour for a public carrier to suddenly go dark and head seemingly purposefully in another direction without a mayday.
We aren't privy to any corroborating satellite data other than what has been released by Inmarsat, nor are we privy to radar data from other countries. I think it best to leave super conspiracy theories of a Malaysian shoot down to one side - there are other more likely (but still not palatable) reasons the aircraft was diverted.