Place bacon in a large skillet over medium-low heat; cook until just crisp, about 10 minutes. Transfer bacon to a paper towel-lined plate, reserving bacon grease in skillet.
Increase heat to medium-high under skillet; saute Brussels sprouts in the bacon grease until lightly browned, 2 to 3 minutes. Add garlic to Brussels sprouts; saute until garlic is fragrant, 1 to 2 minutes.
Pour chicken stock over Brussels sprouts mixture and cover skillet with a lid; simmer until Brussels sprouts are bright green, 4 to 5 minutes. Remove lid and continue simmering until liquid is evaporated and sprouts are tender, about 5 more minutes.
Remove skillet from heat and stir bacon, butter, vinegar, salt, and pepper into Brussels sprouts mixture until butter is melted.
