Make the picadillo: Heat oil in a large skillet over medium-high heat. Add onion, and cook until just starting to turn translucent, 3 to 5 minutes. Add beef, and cook, breaking up pieces with the back of a wooden spoon, until beef has browned, about 4 minutes. Spoon out excess fat so that 1 tablespoon remains. Stir in garlic, tomatoes, paprika, chili powder, oregano, salt, pepper, and cumin. Add water, and bring to a boil. Reduce heat to medium, and simmer until thick and very little liquid remains, about 18 minutes. Stir in vinegar.
Make the tacos: Heat 1 inch oil in a heavy-bottomed, high-sided skillet until a deep-fry thermometer registers 325 degrees. Fry tortillas, one at a time, by folding in half and laying one half flat in oil while holding other half with tongs, and repeating with the other half to form a half moon shell, until golden and crisp, 1 to 2 minutes per half. Meanwhile, let finished taco shells rest on a wire rack set over a rimmed baking sheet.
Spoon picadillo into each taco shell, and garnish with lettuce, onion, shredded cheese, and salsa picante.
Store-bought taco shells can be substituted for homemade ones.