Heat over medium just until bubbles appear on bottom; stir in 1 tablespoon vinegar.
Poach eggs.
In a medium saucepan, cook bacon over medium-high heat, turning occasionally, until browned, 4 to 6 minutes; use a slotted spoon to transfer to a paper towellined plate.
Cook shallot in rendered bacon fat until soft, about 2 minutes.
Carefully add remaining 1/2 cup vinegar, and boil over high heat until reduced to 1/3 cup, 2 to 3 minutes.
Season with salt and pepper.
In a large bowl, toss spinach with bacon and hot vinaigrette.
Divide among four plates; top each with a poached egg.