Don't go by any time lines specifically given with a recipe in regards to primary fermentation. Each batch of beer and each fermentation is its own animal meaning type of yeast, gravity of the beer, temperatures and so on will all affect the performance of the yeast in primary. I say all that to say this take hydrometer readings. It is the only way to know when it's time to rack to secondary. You could add the vanilla to primary and let it sit, however if you're going to secondary it I would wait until primary is complete (by hydrometer readings), transfer to secondary, and add the vanilla at that point and age from there. Hope that helps, good luck.