Preheat grill and fish basket until they're both very hot.
While grill heats up, heat the olive oil on medium-high in a sauté pan. When the oil is hot, and ripples when swirled around the pan, add the plums and stir with a wooden spoon. Sauté about 2 minutes, until the plums soften.
Add the soy sauce and cayenne, and continue sautéing for another 2 minutes. Once the sauce has thickened, turn the heat to low, and add the vinegar. Remove the sauce from heat, and cover the pan to keep the sauce warm while you grill the salmon.
Pat salmon dry. Sprinkle both sides with salt and pepper.
Carefully open the hot fish basket, using heat proof gloves if your basket doesn't have an insulated handle, and set salmon inside the basket.
Grill the salmon 4-5 minutes, skin side down, then flip the basket. Grill the salmon for about 4 more minutes, until the salmon's translucent pink flesh has just barely turned pale pink. (If you're concerned about overcooking your salmon, you can leave the grill lid off, and wait until the flesh has turned cooked through about halfway before flipping the fish. If you leave the lid off, take care to stand back from the grill, since the oil dripping off this extra-oily fish can cause the grill to flame up.)
Remove salmon from the basket, and serve immediately with the plum sauce.