This modified one should work:

=ROUNDUP(($B2+1-StartDate)/7,0)

I think the reason you are not getting the vlookup you want is that the
above satisfies your requirement and is so much simpler. Maybe someone
will come along and post the vlookup/index/match/offset formula you are
requesting (and if they do it will probably be an array formula), but
the above formula is simple and does what you are looking for.