Slice off the ends of each grapefruit and place the fruit cut side down on a cutting board. Slice off the peel and pith. Working over a bowl, make a cut on either side of each segment to release it from the membrane into the bowl. Squeeze the remaining juice into a saucepan; add the vinegar and honey and simmer over medium-high heat until syrupy. Add to the grapefruit sections.
Meanwhile, preheat a grill pan to medium-high. Add the scallions and cook until slightly charred; cool and thinly slice. Add the scallions and avocado to the grapefruit segments.
Preheat a grill pan to medium. In a bowl, combine the pork and onion powder; season with salt. Shape into 4 patties. Grill the patties, turning once, until cooked, about 8 minutes.
Meanwhile, spread the butter on the rolls and grill until toasted. Place a patty on each roll bottom. Top with the grapefruit-avocado salad and the roll tops.