Seven, in numerology, is a very powerful and magical number. Being born "under the seven" in the old days meant you were destined to be taken into mystical training of some sort...it represents completion and understanding and wisdom and a deep connection to the unseen world (dreams, etc.); hence children whose birthdates corresponded to it were closely watched to see if they had powers or skills that went beyond the usual...I would imagine the seventh son of a seventh son thing is related to this...

I have also heard the vampire thing repeated widely...

one pal of mine is a seventh son; he is a horror writer by trade.

Peg