Remember that the child side of `vfork(2)` can only do async-signal-safe things as `vfork(2)` is documented, and it's really as if it is executing in the same thread as the parent (down to thread-locals, I do believe), so really, `vfork(2)` shouldn't really be all that special for Fil-C! You might want to try it.
As u/foota notes, it's important. The point of `vfork(2)` is that it's _fast_. https://news.ycombinator.com/item?id=30502392