Never used Vanilla in an IPA before, but I used 3 split beans (soaked in vodka for ~3 weeks) in an Imp stout before... let it sit in secondary (poured the beans and vodka in) for about 2 months and the vanilla honestly didn't come through much. Apparently it shows through more with age so I'm hopeful it'll start to come out.. It' been in bottle sfor about three months now.
Good luck.