Place 8 cups of water and the carrot, onion, celery, bay leaves, parsley stems and black peppercorn in a large saucepan and bring to a simmer for 15 minutes.
Add the white wine vinegar and keep at a simmer.
Wash the octopus in plenty of cold water and remove the tentacles with a sharp knife.
Blanch the octopus for about 45 seconds and remove from the poaching and into an ice bath to cool rapidly.
Drain.
Cool the poaching liquid.
Place the octopus in two separate sous-vide bags.
Add a few tablespoons of the poaching liquid to the bags and vacuum-seal on high.
Place the bags in the sous-vide machine for 4 hours at 185°F.
Cool in an ice bath.
Open the bags, discard the liquid and the bags.
Remove the slimy membrane.
Place the octopus in a new bag with olive oil and seal again.
Keep refrigerated until ready to use.
Grill before serving.