This shouldn't really care about VMS here. As for the rest, feel free to propose a patch :)
Please note that most non-trivial system calls, such as ftruncate(), mremap(), even mmap() itself, deserve to be enclosed in Py_BEGIN_ALLOW_THREADS/Py_END_ALLOW_THREADS pairs.

However, you'll have to be careful that the internal state of the mmap object remains consistent, such that using it from several threads doesn't crash the interpreter.
(you can even add multi-threaded unit tests if you want to be sure of this)