...
static union {
    ...
    VkSurfaceFormatKHR sf[BUFFERSIZE/sizeof (VkSurfaceFormatKHR)];
    ...
} tmpbuffer;

... y después para invocar a la función:

VkSurfaceFormatKHR buff = tmpbuffer.sf;
int len = BUFFERSIZE/sizeof (VkSurfaceFormatKHR);
VkResult res = vkGetPhysicalDeviceSurfaceFormatsKHR(dev, surf, &len, buff);
if (res != VK_SUCCESS || VK_INCOMPLETE) {
    /* manejar errores */...
}
if (res == VK_INCOMPLETE) {
    buff = malloc(len * sizeof (VkSurfaceFormatKHR));
    vkGetPhysicalDeviceSurfaceFormatsKHR(dev, surf, &len, buff);
}

[..] usarlo [..];

/*liberarlo:*/
if (res == VK_INCOMPLETE) free(buff);