Committed a test case for that, commit ID 898a3b520202dc904f4dbd2863380842bcf0e727
I'm not sure if this is bug or missing feature, whether it's supposed to have
its own VforkObserver, or it should share the observer with Fork or CloneObserver.
(Btw, I tried to use CloneObserver, but it doesn't provide vfork info either.)
*** This bug has been marked as a duplicate of 1583 ***