SpaceX uses VxWorks for their hard real-time requirements: http://blogs.windriver.com/vxworks/2010/12/vxworks-helping-c...

They use Linux for non-hard-real-time stuff. Their microcontrollers also do not use Linux.

Coincidentally, Tesla uses QNX.

Yes for their microcontrollers they use VxWorks. But Linux is still used for the flight software (i.e. mission control).

VxWorks does not run on microcontrollers.

Don't know your definition of microcontroller but VxWorks runs on e.g. Cortex-M3/4, MIPS, 68K, SH, SparcV8, PowerPC thus including some devices I could microcontrollers. But there is no clear cut definition of microcontroller, microprocessor, SoC, whatnot.