Heat grill or grill pan to medium. In a shallow bowl, combine the chili sauce, sugar, fish sauce, and garlic. Add the steak and turn it to coat.
In a medium saucepan over medium-high heat, bring the vinegar to a boil. Remove from heat and toss in the radishes, carrot, and bell pepper.
Grill the steak to the desired doneness, 4 to 6 minutes per side for medium-rare. Let rest 5 minutes before slicing.
Grill the bread, cut-side down, until slightly charred, about 1 minute.
Cut the baguette crosswise in half and spread with the mayonnaise. Form 2 sandwiches with the steak, vegetables, and cilantro.