In a large bowl, toss the cucumber, carrot, radish, mint, cilantro and scallions with the fish sauce, salt and lime juice. Let the slaw stand 10 minutes.
Preheat a grill to medium-high heat. Butterfly the hotdogs by slicing lengthwise about three-quarters of the way through. Grill the hot dogs until browned and a little crispy, about 3 minutes per side. Butter one side of each baguette piece and toast until golden brown.
Spread 1 tablespoon of the mayonnaise on one side of 4 of the baguette pieces. Put a hot dog on top of each, followed by some slaw, torn basil leaves and jalapeno slices. Top with a remaining baguette piece.