Sending signals internally such as from with abort or pthread_kill goes to the wrong process and gives errors. See the code below. I know vfork is hard to support but as of now posix_spawn simply isn't a mature enough replacement.

#define _GNU_SOURCE

#include <assert.h>
#include <sched.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>
#include <sys/wait.h>
#include <unistd.h>

static int fork_routine(void *args);
static size_t small_stack_size(void);

int main(void)
{

	size_t page_size = sysconf(_SC_PAGE_SIZE);
	size_t stack_size = small_stack_size();

	void *stack = mmap(0, stack_size + 2U * page_size, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS | MAP_STACK | MAP_GROWSDOWN, -1, 0);
	if (MAP_FAILED == stack) {
		perror("mmap");
		return EXIT_FAILURE;
	}

	if (-1 == mprotect(stack, page_size, PROT_NONE)) {
		perror("mprotect");
		return EXIT_FAILURE;
	}

	if (-1 == mprotect(((char *)stack) + page_size + stack_size, page_size, PROT_NONE)) {
		perror("mprotect");
		return EXIT_FAILURE;
	}
	fprintf(stderr, "before fork\n");

	pid_t child = clone(fork_routine, ((char *)stack) + page_size + stack_size, SIGCHLD | CLONE_VM | CLONE_VFORK, 0);
	if (-1 == child) {
		perror("clone");
		return EXIT_FAILURE;
	}
	siginfo_t info;
	if (-1 == waitid(P_PID, child, &info, WEXITED)) {
		perror("waitid");
		return EXIT_FAILURE;
	}

	fprintf(stderr, "out of fork: %i\n", getpid());
	getchar();
	abort();
	return EXIT_SUCCESS;
}
static int fork_routine(void *args)
{
	write(STDERR_FILENO, "in fork\n", sizeof "in fork\n" - 1U);
	_Exit(EXIT_SUCCESS);
}

static size_t small_stack_size(void)
{
	long maybe_page_size = sysconf(_SC_PAGE_SIZE);
	assert(maybe_page_size >= 0);

	long maybe_stack_min_size = sysconf(_SC_THREAD_STACK_MIN);
	assert(maybe_stack_min_size >= 0);

	size_t page_size = maybe_page_size;
	size_t stack_min_size = maybe_stack_min_size;

	return stack_min_size + 3U * page_size;
}