As a rule anything aromatic and delicate is best added as late in the process as possible. Anything expensive is also best added once you've had the majority of losses. In the boil not only will you volatilise a lot of the aroma, you'll have losses to hops, trub, pipework, co2 during fermentation, yeast, sediment and so on all the way up to packaging. I'd rather all my (now very expensive vanilla) goes into the packaged beer! Chop the pods into 1-2mm pieces (I grab a handful and snip them with scissors like chives) cover them with vodka or similar and let steep for a day or two somewhere dark and out of the way (alcohol will help sanitise and also extract the flavour) then add the liquid on packaging (keg or bottling bucket). You could add the liquid and the solids to the fermenter at the end of fermentation prior to cold crashing if it suits, but you'll lose a little flavour to the sediment mitigated perhaps by the bits going into the fermenter. I used to put my spent pods into fresh vodka as you usually get more extraction over time and I'd usually find a use for it.