it also depends on how long your gonna lager it,if its gonna be another 2 months you might want to just leave it,but if your not and want more vanilla flavor add them other beans.
i did the beans in the boil ...no flavor what so ever.then i did the vanilla beans in primary first sip you could barley taste it,second drink the flavor was gone. then next time i put the beans with out any alcohol in the secondary and lagered 2 months and you could taste it pretty good. myself...
did you slice the beans down the center and scrape the insides of the beans? also you don't have to put the beans in alcohol,theres no danger of adding them in the secondary! with 6 beans you should be tasting it good.