Core pears from the bottom, using a small melon baller to scoop out the seeds. Cut a round of parchment to fit inside a large pot (it should be wide enough to hold the pears in one layer). Place pears in pot along with wine, brown sugar, and enough water to just cover pears (2 to 3 cups). Scrape vanilla bean seeds into pot, then drop in pod. Cover with parchment round.
Bring to a boil, then reduce heat; simmer, turning pears occasionally to coat with liquid, until very tender but not falling apart, 10 to 15 minutes, depending on ripeness and size of pears. Use a slotted spoon to transfer pears to a large bowl. Remove and discard vanilla bean pod. Cover bowl with parchment round (or cut a new one to fit).
Bring poaching liquid to a boil and cook until reduced to consistency of thin syrup, about 10 minutes. Pour syrup over pears and refrigerate, covered with parchment, until cool. Serve pears in shallow bowls or cups; tuck a chocolate-mint leaf into stem end of each one, if desired.