I think the -vpython-root description is wrong:

  -vpython-root string
    	Path to virtual environment root directory. Default is the working directory. If explicitly set to empty string, a temporary directory will be used and cleaned up on completion. (default "/usr/local/google/home/ayatane/.vpython-root")

From general usage, it appears that the default is the temporary directory mentioned, not the working directory.  Also, the default behavior is to not clean up said temporary directory.