Python needs to be registered as an Active Script / Windows Script Host engine to use it from VBP scripting [1]. I believe ActivePython [2] must be used, including the 'Python for Windows Extensions' option when installing.
Also, VBP will not capture language-specific output (i.e., print) statements; you would need to use WScript.Echo or Builder.LogMessage to send output to VBP. And you do need to double up bracket characters so they will not be interpreted by VBP as a script expression [3].
Another option would be to invoke Python directly (not using Active Scripting) from a Run Program action.
[1]
http://www.visualbuild.com/Manual/script.htm
[2]
http://www.activestate.com/Products/ActivePython/
[3]
http://www.visualbuild.com/Manual/scriptexpressions.htm