I think it's less of an issue now than say 10 years ago. VxWorks 6.x added support for protection domains (MPU/MMU support) and RTPs (real time processes). VxWorks 5 everything operated in the kernel. Even with 6.x very little typically runs by default in user space on a VxWorks setup.

With respect to the message passing - both support messaging. VxWorks has several types of message queues - vxworks proprietary msgQLib API, POSIX api etc. QNX has much the same MsgSend/MsgRecv which is the microkernel API and POSIX. QNX has an add-on PubSub middleware that the OP of the usenet group may be thinking of.