I generally grind the nibs coarsely and put it in vodka for a couple of days or more.
If you use vanilla, I would advise you to use the pure stuff. No imitation. It costs more and there is a reason for that. I usually buy vanilla beans (they come in 2 in the tube). Split them lengthwise with a knife, scrape out the vanilla with a spoon and put in a glass container, then chop up what's left of the beans,put that in the same container. Add vodka for several days. Dump it in when you're ready.
I've never used both chocolate and vanilla in the same recipe.
WY1084 is a great choice for yeast for this style.
The recipe says to boil for 90 minutes. I wouldn't boil for 90 mins. but that's just me.
BTW, I'm not sure how long you need to boil for extract, I would probably follow the boil time of a kit used previously.