VBP inherits its environment from the process that started it (unless the starting app explicitly passes a different environment [1], which is rare), and any processes started by VBP will inherit its environment (which could be modified within VBP via the Set Macro action configured to add to environment variables [2]).
Like most apps (for instance, cmd.exe, Visual Studio, etc., Explorer.exe is one of the few exceptions that does), it does not reload its environment from external changes after startup.
[1]
http://msdn2.microsoft.com/en-us/library/ms682425.aspx
[2]
http://www.kinook.com/Forum/showthread.php?threadid=52