Change is really easy, basically you create a new database and use the impex (importing software) to import your phpbb into a brand new vBulletin installation.
Not to sure on the apache vs lighttpd thing, I've heard some information about lighttpd working great with vBulletin, not sure how hard it is to switch an existing apache installation over to lighttpd or vice versa.
Really, I'm going to say stick with Debian, It might be a pain to switch to a different OS, but if you do want to change FreeBSD is a great OS to use.
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