The easy way and the cheap way would be to brew up a cream stout then on bottling day pull off a pint or two and start experimenting with different amounts of extract until you get what your looking for then simply scale up the amount for the rest of the batch. You should probably start at 5 mls per liter or so. It would be a good idea to make a dilution of the vanilla to make it easier to measure such small amounts. If your lazy simply stir in a Tbs of extract into your bottling bucket and taste, then keep adding till you get what you want. Vannila beans are great but very pricey and I imagine you would want to boil them to extract as much flavor as possible or maybe steep them at the end of the boil or even use them as a dry hop. You could buy a sixpack of stout and experiment with that beforehand. Good luck.
"You are what you is." FZ