Thanks for all your research and reference links on this!  As a _posixsubprocess maintainer, I am not against either posix_spawn or vfork being used directly in the future when feasible.

A challenge, especially with platform specific vfork, is making sure we understand exactly which platforms it can work properly on and checking for those both at compile time _and_ runtime (running kernel version and potentially the runtime libc version?) so that we can only use it in situations we are sure it is supposed to behave as desired in.  My guiding philosophy: Be conservative on choosing when such a thing is safe to use.