There really isn't a clear cut difference. Its basically the different amounts of features each forum has and personal preference. Also the phpbb is developed entirely by people who volunteer their time to create the software. vB is developed by a company with paid workers.

Heres a list of feature differences (Note this is version 3.0 which is not the version available in cPanel, you would have to manually install this).
http://www.phpbb.com/about/features/compare.php

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