i found this at jedinet.com
it says that the jedi prescence after the death only has happened when the skywalker family is in trouble and disturbed.
it is the only link between the new quigon´s voice appearance and the last yoda-anakin-ben appearance.
it says that this jedis had to have a real close connection with the chosen one or even to luke who is also a skywalker to be present after death.
i find this theory with many flaws, it doesn´t seem to be the ultimate answer to the enigma
but its a nice theory
head over to this link : http://www.jedinet.com/prequels/fan_scenes/#61205,20020423,3214
and it is about in the middle.
i think it might be close. yoda says in empire, that the force allows you to see things yet to come and friends long gone. or something like that. yoda seems to be the srongest jedi at seeing the future, maybe the skywalkers are so strong that they can materialize their old friends from the force when they're in need. but that doesn't explain the disappearence thing. hopefully we'll have more info in about two weeks.
i never took that line as completely literal. it means luke, not necessarily ob1 as a ghost. as evidenced by the fact that ob1 waited untill luke was watching to die. as for that skywalker theory, i see your point about vader, but maybe its something that they can't control. luke never really appears to try to conjure up ben, except after the duel with vader and he can't (if you face him now i won't be able to help, or whatever). so that being the case vader wouldn't understand the comment if meaning the ghost thing. he definetly doesn't know about luke yet, so i believe thats why he wouldn't understand. he thinks that he just killed the last jedi.