Look, as i stated earlier Vince is an eccentric character. Getbig has become his family. Turning up to the funeral is what family do, albeit 'an internet family'.
Showing up, uninvited, with camera in hand to the funeral of a grandfather that is related to an x-Pro (Lee; he use to post here) is unusual and out of the ordinary.
In summary, Vince believes in his mind he is 'related' because he is part of the Getbig-family! And yes, family is family!