The vfork syscall first appeared in BSD 3.0 and did it the other way around. When calling vfork, the parent is suspended until the child terminates or calls exec. The child is given read-only access to the parent's memory space, or it traps on an attempt to write.

This is how they optimised it in the days before COW, by fully preserving the intended behavioural semantics of fork/exec. You shouldn't modify memory in the child and the OS therefore shouldn't copy the memory space.