Use sharp knife to halve each hard-boiled egg, slicing horizontally through center of egg as if slicing hamburger buns. Place egg halves on cutting board or plate. Wipe off knife blade with paper towels to remove any excess egg yolk, then repeat process with remaining eggs until all hard-boiled eggs have been halved.
Gently scoop yolks out of egg whites and into mixing bowl. Be careful not to tear egg whites. Return empty egg whites to cutting board or plate.
Once all egg yolks have been transferred to bowl, stir and mash yolks to desired consistency. For smoothest filling texture, mash yolks until no solid pieces remain.
Add dijon mustard, vinegar, mayo, softened butter, and salt to bowl. Stir well until ingredients are fully combined, then taste mixture and adjust ingredients as needed to achieve desired flavor. Note: if preparing filling in advance, stop here and cover bowl with lid or plastic wrap. Refrigerate filling no more than 24 hours before filling egg whites. Refrigerate empty egg whites in separate airtight container.
When satisfied with flavor, transfer yolk mixture to pipping bag fitted with piping tip. Work yolk mixture down to piping tip, then twist top of piping bag closed.
While gently applying pressure from back of filled piping bag, pipe yolk mixture directly into cavity of egg whites, swirling filling as if frosting cupcakes. Repeat until all egg whites have been filled.
Dust each deviled egg with smoked paprika to taste, then garnish with crisp bacon pieces. Serve immediately.