Generously butter the inside of a 6-quart slow cooker crock.
Heat the oven to 350°F. Spread the bread cubes on a large rimmed baking sheet. Toast, turning the cubes twice, until crisp, about 8 minutes. Set aside.
Cook the bacon in a 12-inch skillet over medium heat, turning occasionally, until browned and crisp, about 8 minutes. Transfer to paper towels to drain. Crumble the bacon into the slow cooker. Add the bread cubes, and toss well.
In a large bowl, whisk the eggs, milk, maple syrup, and vanilla until smooth. Pour over the bread cubes and bacon, using the back of a wooden spoon to press the top bread cubes into the liquid to moisten them. Let stand in the refrigerator, covered with plastic wrap, overnight.
Remove from the refrigerator and gently toss with a rubber spatula. Cook on low until set, about 2-1/2 hours. Serve by scooping out large spoonfuls.