Yes, fork can fail and we ran into this a few years ago at MemSQL. The problem was that MemSQL would allocate a lot of memory and linux wouldn't allow to fork such a process. A remedy to that is to create a separate process and talk to it via tcp. This small and low on memory consumption process is responsible for fork/exec paradigm.
Why doesn't it use vfork() for fork+exec? AFAIK vfork() doesn't clone the allocated memory of the parent process and is only useful for calling exec immediately after forking.
http://linux.die.net/man/2/vfork
