The vfork() function (which should be called only from a single-threaded process) is useful when the purpose of fork() would have been to create a new system context for a call to one of the exec*() functions.
The vfork() function differs from fork() in that the child doesn't get a copy of the calling process's data. Instead, it borrows the calling process's memory and thread of control until a call to one of the exec*() functions is made. The calling process is suspended while the child is using its resources.
The vfork() child can't return from the procedure that called vfork(), since the eventual return from the parent vfork() would then return to a stack frame that no longer existed.