I did some more research:
* <https://sourceware.org/bugzilla/show_bug.cgi?id=10354> seems to indicate that glibc switched to a different implementation of posix_spawn that uses clone(2) instead of fork()/vfork() to avoid some problems with vfork().
The start of the issue also contains some information on why glibc is (was?) so conservative about using vfork, and a possible work around (disable cancelation points around the call to posix_spawn).
* <https://github.com/lattera/glibc/blob/master/sysdeps/unix/sysv/linux/spawni.c> is the source for the guts of posix_spawn for some version of glibc, and that indeed does use clone(...|CLONE_VFORK...) unconditionally and does not appear to test for POSIX_SPAWN_USEVFORK.
It looks like the advise to use POSIX_SPAWN_USEVFORK is outdated, although I'm not 100% sure of my conclusion. A glibc expert should be able to confirm or refute this.
@pablogsal: do you have more information on why you want to enable this flag? Do you have measurements that show that adding this flag helps?