Trying something much like this currently. Added .5lb lactose and forgot the irish moss (oh well...) plan on having in primary 2-3 weeks, racking into bottling bucket with priming Honey (local stuff, makes for a nice flavor) and then adding 2-3oz's homemade bourbon vanilla extract. I sat down with some stouts from a previous recipe and played with bench trials, adding a little vanilla and lactose to the glass before pouring it. Once the dust had settled (or the head, I should say) I had a stout that had a fantastic almost cream-soda-like taste and good mouthfeel, with a heavenly aroma. This was of course freshly added to the brew so adding before conditioning could have some different results. Using vanilla is interesting because it starts out very pronounced when young (I've made mead and wine longer than I have beer ) and has a tendency to become less noticeable over time as it's meshes with/enhances the flavors. I imagine putting a good dose in at bottling followed by 2-3 weeks or more of conditioning will get a good representation out of vanilla without it being predominant. But then again this is an experimental batch, and may have unknown consequences, such as making something so fantastic I'm doomed to repeat it over and over for family and friends.