Haven't had personal experience myself, but a mate of mine has made a chocolate stout a few times that calls for vanilla beans. He uses a decent, but not too expensive vodka around 40% abv. He soaks the pods for at least a week in the vodka and adds it along with the vodka used to soak the pods to primary 3 weeks after pitching yeast. No infection or over carbonation. perhaps you did not soak the vanilla long enough or had some sugar priming issues? Maybe you could somehow boil the vanilla for 5 minutes in water, drain, and then soak in vodka or rum?