Combine the vinegar, water, and salt in a non-reactive pot and bring to a boil over high heat. 

Divide the peppercorns, mustard seeds, garlic cloves, and bay leaves between two sterilized pint jars (see note). Snugly pack the Brussels sprouts into the jars, then pour the hot vinegar mixture over them. Gently tap the jars to remove any air bubbles. Wipe the rims with a clean damp cloth, and apply the lids and rings.

Submerge the jars in boiling water and process for 10 minutes. Remove from the boiling water and let cool on a folded towel on the counter. Allow to cool, then check the seals and store in a cool, dark spot (unsealed jars should be stored in the fridge). Allow to rest for 48 hours before eating them. Sealed jars will keep up to a year in the pantry.