What's the recipe? If it's a really thick, dark stout, with lots of flavorful malts, you'll need more for it to come through than if it's on the lighter side. If you have beans, they're great. Chop up 1-3 (depending on amount of vanilla you want (more if you want A LOT of vanilla) and the recipe you used) and scrape the beans. Either just add to secondary, or soak in a dash of vodka during primary fermentation to extract the flavor, then toss the whole thing in the secondary, beans, pulp and all. 2 weeks seems like a good amount of time to me.
Search for chips vs cubes, and you'll get all kinds of opinions. Just don't over do it.