IMHO, you should look at the recipes section for threads on chocolate/cacao stouts and follow that advice. There's two aspects to consider: quantity of adjuncts and time in the beer.
I don't work with cacao nibs, but I do know about 4 oz of cocoa powder to a six gallon batch added at 5 minutes to the boil is just about perfect for that chocolate hit in my stouts. Vanilla bean is best split down the middle, scraped out and add the scrapings and the rest of the split bean to the secondary (more surface area exposure). I find that about 5 days is all you need for a nice vanilla flavor.
You can use a weighted hops bag (put a sanitized quartz rock in it) and sink it in your primary/secondary with your adjuncts in it, and let it sit. Try a taste test at 3-5-7 days and see how it's tasting. You might wait until it develops just slightly more flavor than you'd like as these flavors will mellow out a little with 3 weeks bottle conditioning or more.
Beer so good, it's frightening.
2014: Seven Heavens Series One: Vilon, Mystic Melomel, Skeeter Pee.