Combine sushi vinegar, water and sugar in a stainless steel saucepan over medium heat, bring to a boil, and stir until the sugar is dissolved, about 1 minute. Allow the mixture to cool and season to taste with salt and pepper. In a stainless steel bowl, combine cauliflower and carrots, and pour in the cooled vinegar mixture. Allow to stand for at least 1 hour. Drain off excess vinegar mixture after the vegetables have marinated.
Preheat oven to 325° F (165°C). Simmer red wine in a large saucepan over medium-high heat until reduced by half. Meanwhile, combine flour, salt, and pepper in a large bowl. Dredge oxtail in the seasoned flour and shake off excess; set aside. Heat butter in a roasting pan over medium-high heat and brown the oxtail pieces on all sides, about 10 minutes. Remove oxtails and set aside.
Melt more butter in the saucepan. Stir in shallots, garlic and mirepoix. Cook, stirring, until vegetables have softened, about 10 minutes. Add wine, deglaze and reduce by half. Stir in thyme, bay leaf, parsley, beef broth and soy sauce. Place oxtail on top of vegetables in a single layer and bring to a boil.
Cover saucepan with a tight fitting lid or aluminum foil and bake in preheated oven until oxtail is very tender and nearly falling off the bone, approximately 3 to 3 1/2 hours. Remove oxtail to a serving dish, cover, and keep warm. Pull meat off the bones with a fork and reserve.
Strain braising liquid through a mesh strainer into a saucepan. Simmer over medium-high heat until liquid has reduced to 2 cups. Whisk in butter to finish and pour over the oxtail.
To assemble Bahn Mi Sandwich to order, slice baguette in half lengthwise and pull out some of the crumb, leaving a cavity for the filling. Place baguette halves under the broiler for 1-2 minutes or until lightly toasted. Spread each half of the baguette with mayonnaise and fill the cavity of the bottom half of the bread with oxtail, cucumber, Pickled Cauliflower, cilantro sprigs, jalapeño pepper, grapes and Brie. Broil until cheese melts. Drizzle with Ponzu Dressing and top with the other half of the baguette.