Be sure to use cold eggs, straight from the refrigerator, because they will be easier to break and are more likely to hold their shape well during cooking.
Fill a large skillet with 1 1/2 inches of water. Heat over medium heat just until bubbles appear on bottom; stir in 1 tablespoon vinegar.
Break one egg into a cup; dip cup in water 20 seconds, then release egg. Repeat with remaining eggs. Cook until whites are just firm, 4 to 6 minutes. Remove with a slotted spoon. Drain on paper towels; trim edges with a knife.
In a medium saucepan, cook bacon over medium-high heat, stirring occasionally, until browned, 4 to 6 minutes. Remove, and drain on paper towels. To the same pan, add shallot, and cook in bacon fat until soft, about 2 minutes. Add remaining 1/2 cup vinegar; boil over high heat until reduced to 1/3 cup, 2 to 3 minutes. Season with salt and pepper.
In a bowl, toss spinach with bacon and hot vinaigrette. Divide among plates; top each with a poached egg.